backup all your personal data to a DVD, or whatever drive first, like a thumb drive...
reformat the drive and load a fresh operating system, you will need the operating system installation disk.
See the instructions at the top of this forum: "how to repair install windows XP."
this gives you the directions for starting the installation.
When you get to the selection, select new install instead of repairing the old one...
this wipes the entire drive and will install a fresh OS on the drive.