first you want to back up all your personal files to a thumb drive etc...
then you need the XP install disk, and I would recommend SP3 pro, on a disk.
while you start the computer hold down F8, before windows starts you should see a menu come up (on some computers, the function is another number, and not F8, seen the manual for your computer, or download the full motherboard manual from the manufacturer)
put the disk in the drive
use up / down arrows to select "boot from disk drive"
push enter, follow along with the directions on the screen to reformat and install XP. This will erase everything on the drive, which is why you need to back up any files before reformatting the drive.