I have been pointed to this article at Microsloth - Install and Use Windows XP mode in Windows 7
I'm doing it because I have about 3 applications that I miss terribly, and a scanner that doesn't want to work in Win7 because it's 16bit.
(And I don't see why I should have to upgrade my scanner that I use 1ce a year, just because Microsoft. It's Old. It works! Phfft)
So I've just Installed it, and Windows Virtual PC, and playing with it now.
It's lovely seeing the old XP mode screen again!