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.
The PC should have come with a set of restore disks, you can use those to get everything back the way it was when it was new. Need to backup all your files and settings though or you'll lose it all during the restore.