Do you have a Windows XP reinstall/install disc? If you don't.... I'm not sure if there's another way, but I might be incorrect. If you do:
Open up BIOS and set it to boot from the CD drive.
Then, open the disc drive and insert your reinstall/install (will an install disc work? I don't know) disc and close the disc drive.
Now, save changes and close BIOS and the computer should restart.
When it restarts, just wait and the CD should spin up and the computer will load the disc.
Now, follow the on-screen prompts, and it should guide you through the process of reinstalling.