Unfortunately, you will need a XP CD to format your hard drive
If you do not have a CD you may be able to reinstall XP from a burned CD. If you have a sticker on the side of your machine with the product key then you can use another disk. Just make sure it's the same version (home edition, professional, media center etc..) as the one that you already have on you hard drive.
Do you have your product key?
Do you know anyone with an XP CD?
If you dont have your product key, Windows will let you format(erase) your drive but you cannot reinstall your operating sytem!