Do not buy anything, there is no need!!!
If you already own an XP CD, the procedure is simple.
Place the XP CD in the drive and boot from it. Follow the prompts through for a fresh install and elect to format the drive at the appropriate stage.
If you don't have an XP CD or don't want to install XP, then all you need is a suitable bootdisk. Give more info on what you want to do exactly and I'll supply full instructions.