the answer is by inclusive_disjunction :
Yes. If you really want to, you can "downgrade" from Vista back to XP, providing that either that your computer manufacturer supports it:

http://blogs.zdnet.com/microsoft/?p=543

or you have an original, retail copy of XP. In the latter case, just back up your files, and format the hard drive by booting from the Windows XP CD, and telling it to format the hard drive. In the former, you will have to talk to your computer manufacturers' support staff. They may or may not be able to help you downgrade.