You can put the XP disk in and boot from it, within there you can go through a prompt to format your hard drive and reinstall Windows. It's fairly straight forward if you read whats on the screen. It will wipe all your data off and load up a blank XP format, just a tip, when you choose what to format your partition as pick NTFS (Quick) or you will be there for ages.
/edit You will need your key code for the install as well.