The Differences Between MBR and GPT

If you have dabbled with your hard disk and is always doing formatting and partitioning, you will surely come across the term “MBR” and “GPT”. This is especially evident when you are dual-booting your Mac and faced with the problem of having to switch from GPT to MBR. You probably are wondering, what are the differences between MBR and GPT and is there any benefit using one over the other? We wil clear your doubt in this article.

Hard Disk Partitions

You probably know that you can split your hard disk into several partitions. The question is, how does the OS know the partition structure of the hard disk? That information has to come from some where. This is where MBR (Master Boot Record) and GPT (Guid Partition Table) come into play. While both are architecturally different, both play the same role in governing and provide information for the partitions in the hard disk.

Master Boot Record (MBR)

[mte_content_ads]MBR is the old standard for managing the partition in the hard disk, and it is still being used extensively by many people. The MBR resides at the very beginning of the hard disk and it holds the information on how the logical partitions are organized in the storage device. In addition, the MBR also contains executable code that can scan the partitions for the active OS and load up the boot up code/procedure for the OS.

For a MBR disk, you can only have four primary partitions. To create more partitions, you can set the fourth partition as the extended partition and you will be able to create more sub-partitions (or logical drives) within it. As MBR uses 32-bit to record the partition, each partition can only go up to a maximum of 2TB in size. This is how a typical MBR disk layout looks like:

There are several pitfalls with MBR. First of all, you can only have 4 partitions in the hard disk and each partition is limited to only 2TB in size. This is not going to work well with hard disk of big storage space, say 100TB. Secondly, the MBR is the only place that holds the partition information. If it ever get corrupted (and yes, it can get corrupted very easily), the entire hard disk is unreadable.

GUID Partition Table (GPT)

GPT is the latest standard for laying out the partitions of a hard disk. It makes use of globally unique identifiers (GUID) to define the partition and it is part of the UEFI standard. This means that on a UEFI-based system (which is required for Windows 8 Secure Boot feature), it is a must to use GPT. With GPT, you can create theoretically unlimited partitions on the hard disk, even though it is generally restricted to 128 partitions by most OSes. Unlike MBR that limits each partition to only 2TB in size, each partition in GPT can hold up to 2^64 blocks in length (as it is using 64-bit), which is equivalent to 9.44ZB for a 512-byte block (1 ZB is 1 billion terabytes). In MicrosoftWindows, that size is limited to 256TB.

From the GPT Table Scheme diagram above, you can see that there is a primary GPT at the beginning of the hard disk and a secondary GPT at the end. This is what makes GPT more useful than MBR. GPT stores a backup header and partition table at the end of the disk so it can be recovered if the primary tables are corrupted. It also carry out CRC32 checksums to detect errors and corruption of the header and partition table.

You can also see that there is a protective MBR at the first sector of the hard disk. Such hybrid setup is to allow a BIOS-based system to boot from a GPT disk using a boot loader stored in the protective MBR’s code area. In addition, it protects the GPT disk from damage by GPT-unaware disk utilties.

OS Support

Intel Macs are using GPT by default and you won’t be able to install Mac OS X (without tweaks and hacks) on a MBR system. Mac OS X will run on MBR disk though, it is just that you won’t be able to install on it.

Most Linux kernels come with support for GPT. Unless you are compiling your own kernel and you didn’t add this feature in, you should have no problem getting your favorite distro to work in GPT disk. One thing to note, you wil have to use Grub 2 as the bootloader.

For Windows, only the 64-bit version of Windows from XP onward support booting from GPT disk. If you are getting a laptop pre-installed with 64-bit Windows 8, most probably it is using GPT. For Windows 7 and earlier version, the default configuration will be MBR instead of GPT.

Conclusion

In most cases, you will be fine with either MBR or GPT. It is only in situation where you need to install Windows on a Mac, or when you need to have a partition bigger than 2TB, that you need to use GPT, or convert MBR to GPT. Also, for the newer model of computer that uses UEFI, it will only support GPT.

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10 things you should know about Hyper-V

Hypervisor technology is software on which multiple virtual machines can run, with the hypervisor layer controlling the hardware and allocating resources to each VM operating system. Hyper-V is the virtualization platform that is included in Windows Server 2008. Microsoft also recently released a standalone version, called Hyper-V Server 2008, that's available as a free download from the Microsoft Web site.

As server virtualization becomes more important to businesses as a cost-saving and security solution, and as Hyper-V becomes a major player in the virtualization space, it's important for IT pros to understand how the technology works and what they can and can't do with it.

In this article, we address 10 things you need to know about Hyper-V if you're considering deploying a virtualization solution in your network environment.

Note: This information is also available as a PDF download.

#1: To host or not to host?

Hyper-V is a "type 1" or "native" hypervisor. That means it has direct access to the physical machine's hardware. It differs from Virtual Server 2005, which is a "type 2" or "hosted" virtualization product that has to run on top of a host operating system (e.g., Windows Server 2003) and doesn't have direct access to the hardware.

The standalone version of Hyper-V will run on "bare metal" -- that is, you don't have to install it on an underlying host operating system. This can be cost effective; however, you lose the ability to run additional server roles on the physical machine. And without the Windows Server 2008 host, you don't have a graphical interface. The standalone Hyper-V Server must be administered from the command line.

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Note

Hyper-V Server 2008 is based on the Windows Server 2008 Server Core but does not support the additional roles (DNS server, DHCP server, file server, etc.) that Server Core supports. However, since they share the same kernel components, you should not need special drivers to run Hyper-V.

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Standalone Hyper-V also does not include the large memory support (more than 32 GB of RAM) and support for more than four processors that you get with the Enterprise and DataCenter editions of Windows Server 2008. Nor do you get the benefits of high availability clustering and the Quick Migration feature that are included with the Enterprise and DataCenter editions.

#2: System requirements

It's important to note that Hyper-V Server 2008 is 64-bit only software and can be installed only on 64-bit hardware that has Intel VT or AMD-V virtualization acceleration technologies enabled. Supported processors include Intel's Pentium 4, Xeon, and Core 2 DUO, as well as AMD's Opteron, Athlon 64, and Athlon X2. You must have DEP (Data Execution Protection) enabled (Intel XD bit or AMD NX bit). A 2 GHz or faster processor is recommended; minimum supported is 1 GHz.

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Note

Although Hyper-V itself is 64-bit only, the guest operating systems can be either 32-bit or 64-bit.

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Microsoft states minimum memory requirement as 1 GB, but 2 GB or more is recommended. Standalone Hyper-V supports up to 32 GB of RAM. You'll need at least 2 GB of free disk space to install Hyper-V itself, and then the OS and applications for each VM will require additional disk space.

Also be aware that to manage Hyper-V from your workstation, you'll need Vista with Service Pack 1.

#3: Licensing requirements

Windows Server 2008 Standard Edition allows you to install one physical instance of the OS plus one virtual machine. With Enterprise Edition, you can run up to four VMs, and the DataCenter Edition license allows for an unlimited number of VMs.

The standalone edition of Hyper-V, however, does not include any operating system licenses. So although an underlying host OS is not needed, you will still need to buy licenses for any instances of Windows you install in the VMs. Hyper-V (both the Windows 2008 version and the standalone) support the following Windows guest operating systems: Windows Server 2008 x86 and x64, Windows Server 2003 x86 and x64 with Service Pack 2, Windows 2000 Server with Service Pack 4, Vista x86 and x64 Business, Enterprise, and Ultimate editions with Service Pack 1, and XP Pro x86 and x64 with Service Pack 2 or above. For more info on supported guests, see Knowledge Base article 954958.

Hyper-V also supports installation of Linux VMs. Only SUSE Linux Enterprise Server 10, both x86 and x64 editions, is supported, but other Linux distributions are reported to have been run on Hyper-V. Linux virtual machines are configured to use only one virtual processor, as are Windows 2000 and XP SP2 VMs.

#4: File format and compatibility

Hyper-V saves each virtual machine to a file with the .VHD extension. This is the same format used by Microsoft Virtual Server 2005 and Virtual PC 2003 and 2007. The .VHD files created by Virtual Server and Virtual PC can be used with Hyper-V, but there are some differences in the virtual hardware (specifically, the video card and network card). Thus, the operating systems in those VMs may need to have their drivers updated.

If you want to move a VM from Virtual Server to Hyper-V, you should first uninstall the Virtual Machine Additions from the VM while you're still running it in Virtual Server. Then, shut down the VM in Virtual Server (don't save it, because saved states aren't compatible between VS and Hyper-V).

VMware uses the .VMDK format, but VMware images can be converted to .VHD with the System Center Virtual Machine Manager (referenced in the next section) or by using the Vmdk2Vhd tool, which you can download from the VMToolkit Web site.

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Note

Citrix Systems supports the .VHD format for its XenServer, and Microsoft, Citrix, and HP have been collaborating on the Virtual Desktop Infrastructure (VDI) that runs on Hyper-V and utilizes both Microsoft components and Citrix's XenDesktop.

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#5: Hyper-V management

When you run Hyper-V as part of x64 Windows Server 2008, you can manage it via the Hyper-V Manager in the Administrative Tools menu. Figure A shows the Hyper-V console.

Figure A: The Hyper-V Management Console in Server 2008

The Hyper-V role is also integrated into the Windows Server 2008 Server Manager tool. Here, you can enable the Hyper-V role, view events and services related to Hyper-V, and see recommended configurations, tasks, best practices, and online resources, as shown in Figure B.

Figure B: Hyper-V is integrated into Server Manager in Windows Server 2008.

The Hyper-V management tool (MMC snap-in) for Vista allows you to remotely manage Hyper-V from your Vista desktop. You must have SP1 installed before you can install and use the management tool. You can download it for 32-bit Vista or 64-bit Vista.

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Tip

If you're running your Hyper-V server and Vista client in a workgroup environment, several configuration steps are necessary to make the remote management tool work. See this article for more information.

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Hyper-V virtual machines can also be managed using Microsoft's System Center Virtual Machine Manager 2008, along with VMs running on Microsoft Virtual Server and/or VMware ESX v3. By integrating with SCCM, you get reporting, templates for easy and fast creation of virtual machines, and much more. For more information, see theSystem Center Virtual Machine Manager page.

Hyper-V management tasks can be performed and automated using Windows Management Instrumentation (WMI) and PowerShell.

#6: Emulated vs. synthetic devices

Users don't see this terminology in the interface, but it's an important distinction when you want to get the best possible performance out of Hyper-V virtual machines. Device emulation is the familiar way the virtualization software handles hardware devices in Virtual Server and Virtual PC. The emulation software runs in the parent partition (the partition that can call the hypervisor and request creation of new partitions). Most operating systems already have device drivers for these devices and can boot with them, but they're slower than synthetic devices.

The synthetic device is a new concept with Hyper-V. Synthetic devices are designed to work with virtualization and are optimized to work in that environment, so performance is better than with emulated devices. When you choose between Network Adapter and Legacy Network Adapter, the first is a synthetic device and the second is an emulated device. Some devices, such as the video card and pointing device, can be booted in emulated mode and then switched to synthetic mode when the drivers are loaded to increase performance. For best performance, you should use synthetic devices whenever possible.

#7: Integration Components

Once you've installed an operating system in a Hyper-V virtual machine, you need to install the Integration Components. This is a group of drivers and services that enable the use of synthetic devices by the guest operating system. You can install them on Windows Server 2008 by selecting Insert Integration Services Setup Disk from the Action menu in the Hyper-V console. With some operating systems, you have to install the components manually by navigating to the CD drive.

#8: Virtual networks

There are three types of virtual networks you can create and use on a Hyper-V server:

• Private network allows communication between virtual machines only.
• Internal network allows communication between the virtual machines and the physical machine on which Hyper-V is installed (the host or root OS).
• External network allows the virtual machines to communicate with other physical machines on your network through the physical network adapter on the Hyper-V server.

To create a virtual network, in the right Actions pane of the Hyper-V Manager (not to be confused with the Action menu in the toolbar of the Hyper-V console or the Action menu in the VM window), click Virtual Network Manager. Here, you can set up a new virtual network, as shown in Figure C.

Figure C: Use the Virtual Network Manager to set up private, internal, or external networks.

Note that you can't use a wireless network adapter to set up networking for virtual machines, and you can't attach multiple virtual networks to the same physical NIC at the same time.

#9: Virtual MAC addresses

In the world of physical computers, we don't have to worry much about MAC addresses (spoofing aside). They're unique 48-bit hexadecimal addresses that are assigned by the manufacturer of the network adapter and are usually hardwired into the NIC. Each manufacturer has a range of addresses assigned to it by the Institute of Electrical and Electronics Engineers (IEEE). Virtual machines, however, don't have physical addresses. Multiple VMs on a single physical machine use the same NIC if they connect to an external network, but they can't use the same MAC address. So Hyper-V either assigns a MAC address to each VM dynamically or allows you to manually assign a MAC address, as shown in Figure D.

Figure D: Hyper-V can assign MAC addresses dynamically to your VMs or you can manually assign a static MAC address.

If there are duplicate MAC addresses on VMs on the same Hyper-V server, you will be unable to start the second machine because the MAC address is already in use. You'll get an error message that informs you of the "Attempt to access invalid address." However, if you have multiple virtualization servers, and VMs are connected to an external network, the possibility of duplicate MAC addresses on the network arises. Duplicate MAC addresses can cause unexplained connectivity and networking problems, so it's important to find a way to manage MAC address allocation across multiple virtualization servers.

#10: Using RDP with Hyper-V

When you use a Remote Desktop Connection to connect to the Hyper-V server, you may not be able to use the mouse or pointing device within a guest OS, and keyboard input may not work properly prior to installing the Integration Services. Mouse pointer capture is deliberately blocked because it behaves erratically in this context. That means during the OS installation, you will need to use the keyboard to input information required for setup. And that means you'll have to do a lot of tabbing.

If you're connecting to the Hyper-V server from a Windows Vista or Server 2008 computer, the better solution is to install the Hyper-V remote management tool on the client computer.

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Hyper-V and VMware vSphere Architectures: Pros and Cons

Both VMware and Microsoft have been in the server virtualization scene for a number of years — VMware for more than a decade now, while Microsoft entered into it relatively recently.

It is imperative for IT workers or organizations to understand the differences between the Microsoft Hyper-V and VMware vSphere architectures as well as the advantages and disadvantages each technology offers before they propose the virtualization solutions to their customers or employees — or before using it in a production environment.

There are a number of important components to consider when choosing either VMware vSphere or Microsoft Hyper-V, but from an architecture standpoint of view, the following components play an important role when it comes to choosing the right server virtualization product:

• Device Driver Location in the architecture
• Controlling Layer components
• Hypervisor Layer components

In general, there are three types of virtualization architectures virtualization vendors refer to. They are:

• Type 2 VMM
• Type 1 VMM
• Hybrid VMM

While explaining all three types of virtualization architecture is out of the scope of this article, the one that we'll primarily focus on in this article is the Type 1 VMM. Type 1 VMM is what Microsoft Hyper-V and VMware are using to implement their server virtualization technologies.

Type 1 VMM can be further divided into two subcategories:Monolithic Hypervisor Design and Microkernelized Hypervisor Design. Both designs have three layers in which different components of virtualization product operate.

The lowest layer is called the "Hardware Layer" and is virtualized by the "Hypervisor layer" running directly on top of the "Hardware Layer." The top layer is called "Controlling Layer." The overall objective of the "Controlling Layer" is to control the components running in this layer as well as provide the necessary components for virtual machines to communicate with the "Hypervisor Layer."

Note: The "Hypervisor layer" is sometimes referred as "VMM Layer" or "VM Kernel Layer."

Microkernelized Hypervisor Design

The Microkernelized Hypervisor Design is used by Microsoft Hyper-V. This design does not require the device drivers to be part of the Hypervisor layer — the device drivers operate independently and run in the "Controlling Layer" as shown in the image below:

The Microkernelized Hypervisor Design provides the following advantages:

• Device drivers are not needed for each device to be incorporated in the "Hypervisor Layer" or VMM Kernel
• Since Microsoft does not provide Application Programming Interfaces (APIs) to access the "Hypervisor Layer," the attack surface is minimized. No one can inject foreign code in the "Hypervisor Layer."
• Device Drivers do not need to be hypervisor-aware. So a wide-range of devices can be used with the Microkernelized Hypervisor Design.
• There is no need to shut down "Hypervisor Layer" to include the device drivers. The Device Drivers can be installed in the Operating System running in the "Controlling Layer" (Windows Server 2008, R2, and Windows Server 2012) and used by the Virtual Machines to access the hardware in the "Hardware Layer."
• "Hypervisor Layer" has less overhead for maintaining and managing the Device Drivers.
• The Microkernelized Hypervisor Design allows you to install any other server roles in the "Controlling Layer" apart from Server Virtualization role (Hyper-V).
• There is less initialization time required. The Microsoft Hypervisor code is only 600 KB or so in size.  As a result, the "Hypervisor Layer" does not require more time to initialize its components.

Monolithic Hypervisor Design Page 2

At the same time, there are a few disadvantages associated with the Microkernelized Hypervisor Design that are important to highlight:

The Microkernelized Hypervisor Design requires an operating system to be installed in the "Controlling Layer" before the "Hypervisor Layer" can operate. This is the biggest disadvantage.

• If the operating system running in "Controlling Layer" crashes for any reason, then all other virtual machines will also crash.
• More overhead is required for the operating system running in the "Controlling Layer" to manage the communications between the virtual machines and the "Hypervisor Layer."
• Every organization maintains the security of Windows operating systems by means of applying security updates released by Microsoft. So the operating system running in the "Controlling Layer" must also be applied with the latest security updates. 
  
  As part of the security updates, the Operating System will be rebooted, which in turn requires you to take all virtual machines offline — or move to another node in the cluster without any downtime with the help of Hyper-V Live Migration feature.

Monolithic Hypervisor Design

As you can see in below figure, VMware's vSphere uses the Monolithic Hypervisor Design, which requires the hypervisor-aware device drivers to be hosted in and managed by the "Hypervisor Layer." This is what we see in the "Hypervisor Layer" in the below diagram; the device drivers are part of it:

Hypervisor device drivers must be developed and included in the "Hypervisor Layer" before you can start using the vSphere virtualization product. You cannot run VMware vSphere on hardware that is unsupported.

VMware vSphere operates its components in the "Hypervisor Layer" as shown in the above figure. Product components that it operates are Resource Scheduling, Distributed File System, etc. The Network Stack component, which is responsible for implementing the VMware networking, also operates in the "Hypervisor Layer." The Storage Stack component allows "Controlling Layer" components to access storage devices.

The Monolithic Hypervisor Design provides the following advantages:

• No operating system is required for controlling all the components of the virtualization product. This is the biggest advantage over the Microkernelized Hypervisor Design used by the Microsoft Hyper-V as discussed above.
• No security patches are needed for components running in the "Controlling Layer."

Disadvantages of Monolithic Hypervisor Design:

• VMware's vSphere does not work on hardware that is not supported. However, VMware has developed a list of compatible hardware on which VMware vSphere is able to run successfully. The list can be found at http://www.vmware.com/resources/compatibility/search.php.
• More initialization time is required. In this design, the initialization time required for the VM Kernel depends on the size of device drivers included in it.
• Since device drivers are initialized as part of the "Hypervisor Layer" initialization, any corrupted foreign code injected in the "Hypervisor Layer" can delay the booting/initialization or cause the server to hang in some cases.

Conclusion

In the article we discussed how Microsoft Hyper-V and VMware vSphere operate differently from each other. We explored how each of the virtualization products uses a different virtualization architecture: Microkernelized Hypervisor Design for Microsoft Hyper-V and Monolithic Hypervisor Design for VMware vSphere.

We also covered some of the primary advantages and disadvantages each virtualization product offers, which IT workers or organizations should find valuable when determining the best virtualization product for implementation in their production environment.

Source:1

Hyper-V Architecture

How to Reset Lost Windows Password with Hiren’s BootCD

If you forgot your Windows password you will not be able to log in to Windows. So what to do in this case? Is there a free password cracking software to reset lost Windows password? Answer is yes. In this tutorial we’ll guide you through step-by-step procedure of resetting lost Windows password using a free Linux-based boot utility – Hiren’s BootCD.

Hiren’s BootCD is an ultimate solution to almost all your computer problems. It comes loaded with hell lot of tools. Each of them is powerful and can be really very helpful if used with caution. You can solve nearly all your PC problems like virus infection, hdd failure, data recovery, Hard disk partitioning, password recovery using this CD.

Note: Hiren’s BootCD a customized Linux distribution that you need to go through so many steps on Command Prompt to reset your forgotten Windows password. If you aren’t tech savvy, I recommend you use the Reset Windows Password utility which comes with GUI graphical interface.

How to Reset Lost Windows Password with Hiren’s BootCD?

To get started, download Hiren’s BootCD from here. Once the download is complete, unzip the download file and you’ll find two useful files: Hiren’s.BootCD.15.2.iso and BurnCDCC.exe. Simply burn the ISO image of Hiren’s BootCD to an empty CD using BurnCDCC program.

Boot your locked PC using Hiren’s BootCD that you’ve created. Make sure you have your BIOS set to boot from CD/DVD. Once you boot from the Hiren’s BootCD, you’ll see a list of powerful system repair and maintenance tools. Choose the Offline NT/2000/XP/Vista/7 Password Changer and press Enter.

The screen will display a list of Linux Kernel Boot options. Simply press Enter to continue.

Now the step to choose the correct drive where the Windows is installed. If you have more than one operating system then choose the correct drive and hit Enter. Here I will type “1″ and press Enter, because the /dev/sda1 is the system partition on my hard drive. Next press Enter to confirm that your registry directory is WINDOWS/system32/config.

When it asks you which part of registry to load, type “1″ and press Enter. When the “chntpw Main Interactive Menu” is shown, type “1″ and press Enter to choose the “Edit user data and passwords” option.

Now the screen will display a list of local user accounts existing on your registry file. For example, if you want to edit Windows administrator password, type “Administrator” and press Enter.

The program will display a list of account properties for your user account. To clear or blank your user account password, type “1″ and press Enter. You’ll receive a message that says “Password cleared!”

After successfully resetting your forgotten Windows password, type “!” to close the User Editor Tool.

Now type “q” and hit Enter to close the Offline Password Editor and Registry tool.

Now type “y” and hit Enter to confirm the password change.

Now it will ask you whether you want to use it again or not. Just type “n” and hit Enter.

Remove your Hiren’s BootCD and press ALT + Ctrl + Del to restart your system. You can then log on your computer with a blank/empty Windows password.

Source:

How to Reset Lost Windows Password with System Rescue CD

If you are a little more technical you might want to simply use the excellent System Rescue CD, which is based on Linux.

Note that if you are using standard Windows encryption for your files, resetting the password will permanently disable access to those files. In that case you should crack the password, which is something we’ll cover in an upcoming article.

If you are an Ubuntu user and forgot your password, we’ve covered how to do that as well, either the easy way with the grub menu or alternately with the live cd.

Creating the System Rescue CD

Before you can do anything else, you’ll need to download a copy of the System Rescue CD and mount to VM

Download the System Rescue CD from sysresccd.org

Resetting Your Password

Now that you have your boot CD, you’ll want to boot from it, which will take you to this very informative prompt, with some basic instructions on how to use the CD.

The first thing we’ll want to do is mount the hard drive, using this command. (Note that you might not need to use the –o force argument, it’s only really for when the system didn’t shut down correctly)

ntfs-3g /dev/sda1 /mnt/windows –o force

You can use the df –m command to verify that the drive has been mounted and that it’s the right drive. Note that it’s mounted on /mnt/windows at this point.

Now you’ll want to change directory into the Windows/System32/config directory inside of your windows installation. For mine, the full path was something like this, but it might be different on yours:

cd /mnt/windows/Windows/System32/config

Once you are in that directory, you should see that there is a SAM file, which is where we’ll want to change the passwords.

To change the password we’ll use the chntpw command, and it’s most useful to use the –l argument first to list out all the usernames in the file.

chntpw –l SAM

Now you can add the –u argument with your username, which will end up being something like this command, except you’ll want to replace geek with your username:

chntpw –u geek SAM

This will present you with a wizard type screen:

I’m going to assume that you want to set a new password, so just type “2” at the prompt, add in your password, and remember to use the “y” key when prompted to save.

At this point your password has been changed, so you can issue the reboot command to restart the computer (should take the disc out of the drive)

And now you should be able to login with the new password:

Note that I tested this technique on both XP, Vista and Windows 7 with good results.

Note: You can use same procedure for reset password.

Source: 1

SystemRescueCd Download

Differences between Windows Server 2008, 2008 SP2 and 2008 R2

Windows Server 2008 and Windows Server 2008 SP2 are the same operating system, just at a different service pack level (Windows Server 2008 started at the SP1 level because it was released quite a bit after Windows Vista and SP1 ).

Windows Server 2008 R2 is the server release of Windows 7, so it's version 6.1 of the O.S.; it introduces quite a lot of new features, because it's actually a new release of the system. 


Windows Server 2008 R2 is includes key enhancements related to virtualization, management, IIS, scalability and reliability, and Windows 7 integration

There are also differences at the GUI level, because WS2008R2 uses the same new GUI introduced with Windows 7 (new taskbar, etc.).  

Depending on what kind of applications you're developing, they may or may not encounter problems on different O.S. releases; you should definitely check MSDN.

**Windows Server 2008 is the same codebase bits as Vista. It is available in two flavors 32 bit and 64 bit versions. 

**Windows Server 2008 R2 is the same codebase bits as Windows 7 x64. It is only available in the 64 bit version.

In Simple terms..

Windows Vista SP1 = Windows Server 2008
Windows Vista SP2 = Windows Server 2008 SP2
Windows 7 = Windows Server 2008 R2

please find the link below 

http://www.microsoft.com/windowsserver2008/en/us/whats-new.aspx.

http://technet.microsoft.com/en-us/library/dd335036(WS.10).aspx  ( windows 2008 SP2 )

http://www.microsoft.com/windowsserver2008/en/us/r2-compare-features.aspx ( windows 2008 R2 )
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Differences between Windows Server 2008, SP2, and R2

So what are the differences between win2k8, win2k8 SP2, and win2k8 R2? These naming conventions and differences between versions are a constant cause for confusion. So here's the short take:

Win2k8 was first released with SP1. Later on came Win2k8 SP2.

Win2k8 R2 is the new version of the OS that introduces several new features. It has the look and feel of Win7, it is only x64 bit, and Hyper-V Quick migration (~VMotion) is introduced.

There's no SP2 installed on top of win2k8 R2. R2 is a clean install or you can upgrade from SP2 to R2. In any case, the SP2 will disappear and it will only be called R2.

The reason for pointing this out is that it was a bit different with win2k3. Here, you installed SP2 and then you installed R2 on top of SP2 and the result was win2k3 SP2 R2 - so service pack and R2 at the same time.

Minimum and maximum memory (RAM)

Minimum and maximum memory (RAM) requirements for current Microsoft Windows operating systems (OS) Hyper-V Dynamic Memory configuration guide

Since the availability of Dynamic Memory Hyper-V feature in Windows Server 2008 R2 Service Pack 1, it became of significant importance to know the specifics of minimum and maximum memory (RAM) requirements for Microsoft Windows operating systems (OS). This is not a major issue for non-virtualized computers, as the amount of memory is fairly static over the lifespan of a physical machine [and is usually provisioned for the maximum amount of memory one can afford]. According to Microsoft’s Hyper-V Dynamic Memory Configuration Guide:“With Dynamic Memory, Hyper-V treats memory as a shared resource that can be reallocated automatically among running virtual machines. Dynamic Memory adjusts the amount of memory available to a virtual machine, based on changes in memory demand and values that you specify.” This article is meant to provide a quick reference guide for the minimum and maximum memory requirements of current Microsoft Windows operating systems.

Microsoft Windows Server 2003 / 2003 R2	Minimum RAM	Maximum RAM
Web	256 MB	2 GB
Small Business Server	768 MB	4 GB
Storage Server	256 MB	4 GB
Storage Server Enterprise	256 MB	8 GB
Standard x86 (32-bit)	256 MB	4 GB
Standard x64 (64-bit)	512 MB	4 GB / 32 GB *
Enterprise x86 (32-bit)	256 MB	32 GB / 64 GB
Enterprise x64 (64-bit)	1 GB	64 GB / 1TB *
Datacenter x86 (32-bit)	512 MB	64 GB / 128 GB *
Datacenter x64 (64-bit)	1 GB	512 GB / 1TB *

* The higher value applies to Windows 2003 Service Pack 1 (SP1) and above.

Microsoft Windows Server 2008 / 2008 R2 *	Minimum RAM	Maximum RAM
Web x86 (32-bit)	512 MB	4 GB
Web x64 (64-bit) *	512 MB	32 GB
Small Business Server x86 (32-bit)	512 MB	4 GB
Small Business Server x64 (64-bit)	512 MB	32 GB
Foundation x64 (64-bit) *	512 MB	8 GB
Standard x86 (32-bit)	512 MB	4 GB
Standard x64 (64-bit) *	512 MB	32 GB
Enterprise x86 (32-bit)	512 MB	64 GB
Enterprise x64 (64-bit) *	512 MB	1 TB
Datacenter x86 (32-bit)	512 MB	64 GB
Datacenter x64 (64-bit) *	512 MB	1 TB

* Windows Server 2008 is the last release of Microsoft Windows Server operating system available in both 32-bit and 64-bit versions. Windows Server 2008 R2 is available only in 64-bit editions.

Microsoft Windows Server 2012 / 2012 R2	Minimum RAM *	Maximum RAM
Foundation	512 MB	32 GB
Storage Server Workgroup	512 MB	32 GB
Essentials	512 MB	64 GB
Standard	512 MB	4 TB
Hyper-V Server	512 MB	4 TB
Storage Server Standard	512 MB	4 TB
Datacenter	512 MB	4 TB

* If you create a virtual machine with the minimum supported hardware parameters (1 processor core and 512 MB RAM) and then attempt to install this release on the virtual machine, Setup will fail. To avoid this, allocate more than 800 MB RAM to the virtual machine you intend to install this release on. Once Setup has completed, you can change the allocation to as little as 512 MB RAM, depending on the actual server configuration.

Microsoft Windows XP	Minimum RAM	Maximum RAM
Starter	64 MB	512 MB
Home	64 MB	2 GB
Professional x86 (32-bit)	64 MB	4 GB
Professional x64 (64-bit)	256 MB	128 GB

In this article I am skipping Windows Vista and moving on straight to Windows 7, which is exactly what most corporate users did.

Microsoft Windows 7	Minimum RAM	Maximum RAM
Starter	1 GB	2 GB
Home Basic x86 (32-bit)	1 GB	4 GB
Home Basic x64 (64-bit)	2 GB	8 GB
Home Premium x86 (32-bit)	1 GB	4 GB
Home Premium x64 (64-bit)	2 GB	16 GB
Professional x86 (32-bit)	1 GB	4 GB
Professional x64 (64-bit)	2 GB	192 GB
Enterprise x86 (32-bit)	1 GB	4 GB
Enterprise x64 (64-bit)	2 GB	192 GB
Ultimate x86 (32-bit)	1 GB	4 GB
Ultimate x64 (64-bit)	2 GB	192 GB

The benefits of Hyper-V Dynamic Memory: “Dynamic Memory helps you use memory resources more efficiently. Before this feature was introduced, changing the amount of memory available to a virtual machine could be done only when the virtual machine was turned off. With Dynamic Memory, Hyper-V can provide a virtual machine with more or less memory dynamically in response to changes in the amount of memory required by the workloads or applications running in the virtual machine. As a result, Hyper-V can distribute memory more efficiently among the running virtual machines configured with Dynamic Memory. Depending on factors such as workload, this efficiency can make it possible to run more virtual machines at the same time on one physical computer.”

As a final note — the basic rule of thumb here is to never provision less than the minimum required amount of memory for a specific guest OS.