Do a google search for reinstalling XP and you'll find dozens of sites with full complete instructions for doing this.
Do make sure you have all the necessary disks for reinstalling all you stuff though, you should have the motherboard driver disk, disks for your printer scanner etc.
Oh and is this a branded computer, do you a full windows XP disk or a restore disk. If its a restore disk then windows is on a hidden partition on the hard drive. If you blank the hard drive you will lose windows completely.