Can I go back to Windows XP from Windows Vista?

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.

You have to wipe the drive. You can’t do a Add/Remove Programs to downgrade. So you need a valid copy of XP to install after formating the drive.

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.

Be careful before you do this. Some of the new hardware does not have drivers for XP.

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)