Ask a cracker (person who makes computer viruses, usually reffered to as a hacker, but that's the wrong name for 'em) to give you Vista a virus that totally wipes your hard drive. Â

Or just format it or something, but idk if that destroys the OS.
Also, I think it's possible to have 2+ (yes, two, maybe even more) OS's on the same computer at the same time, if you have enough hard drive space for both of them...
You just get an option while booting up to pick the OS you want to startup...
So maybe try that.
In other words:
1) Turn off
2) Insert windows xp cd
3) Install
:P