The best way that I have found to Totally wipe out windows XP is if you have a windows 98 boot disk.
If you have it, Boot up to it, and choose CD rom support.
Then you will have a DOS prompt.
Type "Format C:" without the quotes.
After that, follow the directions.
To reinstall windows, First, change the BIOS first boot drive (Get into BIOS by either pressing del or F2 during the boot sequence..It'll tell you which to press)
Change the boot drive to your CD-ROM. Save and exit.
After that, restart the computer with the CD in the drive.
Follow all the instructions, and that is it.
(Afterwards, be sure you set the first boot drive back to the floppy drive)
Hope this helps
JABSNH