Start>Programs>Acessories>System Tools>System Restore
Keep in mind, if you do a system restore, the virus is going to be back. The reason is, at certain points, Windows XP takes a "snap-shot" if you will, of the hard drive. This is where you are restoring from. So if you had a virus, and are now rid of the virus, the virus will be back if you do a system restore.
Make sure Windows is up to date. Also, what virus scanner did you use to remove the virus, and what virus was it? Depending on the virus, your issue could be minor or major, and you still could be infected.
EDIT: You never want to do a system restore after you remove a virus. The safest way to make sure this does not happen is to blow out the system restore folder (delete it). You can only do this with certain bootable tools- you can't do it from Windows or Safe Mode....