Boot from your XP disk, then in the windows instalation at the first screen press ENTER (do not press "R" at the first screen!) Then press F8 then to agree with the license agreement, then press "R". Follow the rest of the steps.
To boot from your XP disk, go to your BIOS. you can access your bios by pressing delete when you start your computer. Somewhere in your BIOS there should be a function like " First boot device" Change that so that its boots from the cd. Save and exit.
Hope this helps..