If you did a clean install over Win XP, Vista created a windows.old folder. You can use that folder to get your computer back to your Win XP system. Go to
http://support.microsoft.com/kb/927522 for instructions.
If that's not the case, then you can try an in-place upgrade. Simply start your PC, insert your Vista DVD, run setup and choose upgrade. I suggest you disable or install your virus software. I've had bad experiences when installing software when a virus software s running.