If you have a Windows XP CD put it in your CD Drive and allow it to boot from it. You may need to go into your Bios to configure it to boot from the CD Rom Drive. To do this turn on your PC and start Tapping F2 or the Del key on your keyboard. Once in the Bios use the arrow keys on your keyboard to highlight a tab called "Boot" or something similat and change the boot order so that the CD Rom is first and the hard drive is second etc. Put your XP CD in the CD Drive and restart the PC.
Follow my guide to perform a repair install:
http://www.pchelpforum.com/tutorials...indows-xp.html