Since upgrading Windows 7 or your machine? If it is the latter, my first thought is that the hardware might not support Windows XP mode due to the requirement that VT be built in natively by the CPU. Obviously, this wouldn't be the problem if you are using the same hardware as before. If your CPU doesn't support hardware assisted virtualization, then you can download KB977206 to run Windows Virtual PC without hardware virtualization.
Answer
Since upgrading Windows 7 or your machine? If it is the latter, my first thought is that the hardware might not support Windows XP mode due to the requirement that VT be built in natively by the CPU. Obviously, this wouldn't be the problem if you are using the same hardware as before. If your CPU doesn't support hardware assisted virtualization, then you can download KB977206 to run Windows Virtual PC without hardware virtualization.
http://www.mydigitallife.info/download-kb977206-to-run-windows-virtual-p...
http://download.microsoft.com/download/0/A/3/0A32
Other than that, I'm pretty much drawing a blank. Good luck.