One of the things Win 7 does specificly is run in "XP Mode". Imagine that, it has a Xp Mode, but no Vista Mode...hmmm. That should tell you something in it's-self
yeah, it tells you that it is already running in "vista" mode... Windows 7 runs on vista drivers (vista doesn't really run on XP drivers, and XP doesn't really run on 2000 drivers. Win 7 runs on all vista drivers I've found).
To my understanding, XP 64-bit didn't run in full 64-bit mode the way vista 64-bit did. I don't know the details, but I remember being told it took a few shortcuts by trying to run on XP's services.