ah you have the upgrade, if all you are wanting to do is format the computer clean, with no OS i would advise downloading a program called Gparted and burning it to disk.
Alternatively you can "Press R to repair" on the boot screen and then from the recovery console type "format C:".....at least i think this is in vista...haha havent done it in a long time, but i know XP does
Edit: CRAP hahahah ninja'd yet again! hahahaha