If you have Windows XP installation media and the proper licensing for it, you can use that to create an installation of Windows XP under Client Hyper-V. It's also possible to transfer the VHD (the file that contains the virtual hard disk image for the OS) from a previous installation of XP Mode—again, as long as you still have a valid license for the copy of Windows that contained XP Mode. You can't just swipe a copy from any old installation of Windows 7. You also can't use a Windows XP OEM recovery/installation CD designed to restore a specific system, since that copy of Windows XP is licensed for that particular piece of hardware and no other (not even virtual machines).
Another way to virtualize Windows XP without buying a full-blown license, but only for short periods of time, is to use Microsoft's Internet Explorer Application Compatibility VPC Image. Microsoft provides downloadable system images for Windows XP, Windows Vista, and Windows 7 that contain previous editions of Internet Explorer (IE6 in XP's case) for the sake of backward compatibility and testing. The images in question time out after a certain point, typically 90 days, but they have been refreshed periodically. As of this writing, the current Windows XP image will expire as of Feb. 14, 2013.
Again, due to the licensing restrictions involved, these system images are not intended to serve as full-blown replacements for a properly licensed copy of Windows. But if you just need something running long enough for a quick test or a temporary fix, Client Hyper-V plus a compatibility image should do the trick.
Use Windows 8 Hyper-V to create an app sandbox or test machine
Virtualization makes it possible to sandbox apps—or even whole variant installations of your main OS—for the sake of testing or evaluation. If you have a program you want to try out but are leery of putting it on a production system, stick it in a Client Hyper-V VM. One of the finest analogies I've ever heard for this was that it's like a roll of paper towels: When you're done using that app, you just wipe down the VM or roll it back to a snapshot. You don't even have to uninstall the program in question.
Snapshotting lets you save the state of the system image, so you can revert to it after you're done testing, in much the same way a backup of a system lets you roll back to the point when the backup was made. That said, snapshots of Client Hyper-V VMs are by default stored in the same directory as the VHD file for the VM, so taking too many snapshots can crowd out space that might be needed by the VHD as more items are added.
This matters twice as much if you use that disk for storing other things. You don't want to take one snapshot after another, then have other programs start bombing because they don't have storage space left.
Another thing to keep in mind about snapshots: Snapshots of a VM reduce disk performance for that VM. Each additional snapshot made of a VM creates that much more of a performance bottleneck, since both the snapshot(s) and the underlying VHD have to be processed. If you're using snapshots as a way to keep a clean test machine, use only as many as you absolutely have to. This rule applies for most any use of Client Hyper-V where you're snapshotting, too.
Finally, using Client Hyper-V as a test platform doesn't change the rules about licensing. You'll still need to have a separate license for the instance of Windows you're running. An MSDN or TechNet subscription is one good way to deal with this, since you can use an OS license from the pool given to you for a testing/evaluation install. For your yearly membership fee, you get tons of other software you can evaluate in the same way.
Use Windows 8 Hyper-V to run another OS prepackaged as a VHD
XP Mode isn't the only way you can run a "prepackaged" operating system from a VHD file. Many other Microsoft operating systems are also available in VHD bundles, such as Windows Server 2012. The same goes for many of Microsoft's server-side applications, such as SQL Server and Exchange Server.
Many other vendors' products—whether OSes or products that come deployed as a virtual appliance—are also being made available in VHD editions. Various Linux distributions, for instance, and products like Citrix XenApp are offered as VHD downloads.
The beauty of this kind of prepackaging is that there's nothing to set up on the host system. You only need to download the VHD, create a virtual machine to host it in the Client Hyper-V console, and boot it.
If you've obtained a virtual disk in a format other than VHD, not to worry. Third-party programs can convert virtual disks between formats, including free ones such as StarWind V2V Converter.
Next page: Run an OS from another machine...