Use Windows 8 Hyper-V to virtualize Windows XP (or another earlier version of Windows)
Much to Microsoft's ongoing chagrin, Windows XP has become The OS That Just Won't Die. Many cling to it because it runs with minimal resources. Others stick with it because they have applications or hardware that work with it and nothing else -- all this in the face of a ticking clock to discontinue its support forever!
As of Windows 7, Microsoft's approach to prolonging backward compatibility with XP was XP Mode. This add-on to Windows 7 used one of the spiritual predecessors to Hyper-V, Microsoft Virtual PC, to run an instance of Windows XP in a virtual machine. With the advent of Windows 8, though, XP Mode has been ditched, to the distress of everyone still using it. The issues were as logistical as they were technical: Microsoft only allowed the use of XP Mode with the Professional, Enterprise, and Ultimate versions of Windows 7, because the price tags for those premium editions covered the licensing rights for the edition of XP in XP Mode as well.
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.