Just format your hard drive, then start the intall process with XP Pro. At some point in the installation process, it will tell you that they could not find an authorized OS for upgrade, to insert the CD with the original OS on it. Once you do that, it will check the old OS CD, then prompt you to replace that CD with your XP Pro CD and the installation will continue. Best way to install XP anyway, instead of just doing an upgrade over a previous OS.
Guess I really don't need to remind you to save all your important data files before you start
AMD XP 2600+
SOYO Dragon 400
1 gig 3200 DDR
Pioneer DVD -/+ RW
2-60 gig HD in RAID, 1 60 gig backup