Boot from the XP CD and during the setup it will ask you if you want to install FRESH copy or repair the installation. Choose FRESH and then DELETE the partition when it tells you to. It should be self-explainatory once you enter the setup.
NOTE that you might have to configure your BIOS to boot from the CD instead of the hard drive. Normally holding down DELETE key on keyboard takes you to BIOS if the options needs to be changed. Refer to motherboard's user guide if you can't figure that part out.
Hope that helps. Let me know how things go for you.