if it is a prebuilt, you should have some recovery cds or dvds that came with it or a recovery partion(under my computer). If you have the recovery cds/dvds, just insert the first cd/dvd, reboot, and when the dialog comes up asking if you want to reformat....select reformat
. If you have a recovery partion...you'll have to either select it real fast when the option comes up on boot(i had to do it on my cousins compaq just the other day....wow it took me 5 tries
) or i think some will let you select it under the partion's properties (just look around the partion...there's usually an option for it).
If it was built by you, then do what macdude said
Edit: oh, and don't forget a reformat wipes the boot drive, so back up any data you still want on the drive.