Hi Goose, a few years ago, I did the Vista to XPPro for a niece. She bought a top of the line Dell before starting college. She got to college and found most of the apps that were supplied by the college would not work on Vista. Largest problem was finding the device drivers that would work with XP and the hardware in the Dell laptop. Dell only had Vista device drivers for that particular laptop.
I recently had similar problem with an older desk top PC that I upgraded from XPPro to Win7 Pro. I could not find a Win7Pro video driver that would work with my video card. There is also a MS licensing issue.