Yes. Format hard drive. Re-install XP. I prefer Win 2K Pro, myself. Though, if you formally upgraded, I think there may be an issue with Microsoft deciding your XP license is now void.
If you bought a new PC with Vista preinstalled, some manufacturers have quietly released downgrade CDs – in response to popular demand. Keeps it legal, but it still wipes your disk though. Try checking with the PC maker’s support site.
Sure you can. I did it, and lots of people too. But if you got Vista on a new PC, pay attention to your warranty, since some manufacturers will void it if you change the OS (yeah, it sucks and it’s totally stupid, but they do it anyway)