Zezombia said:Not the easiest method, but:
Look up partitioning your hard drive (it basically splits it, so there's "two separate ones"), then you can install Xp on one partition, and keep your Vista on the other.
Virtual PC is not the answer if you want to use another operating system seriously. It's only good for quick tests to see if something will fuck your computer over or not.
Zezombia said:It can be dangerous though, make sure you look it up properly and note troubleshooting tips.