Windows 7 Upgrade Disc Reformat

Hi, I had windows vista on my computer, then I upgraded it to windows 7 with the upgrade media. Now I changed my mind and want a clean install of windows 7, is it possible to use the upgrade disc to do a clean install with win 7 already on the computer? Or would I have to reinstall vista first, then use upgrade disc? That will suck because my computer didn’t come with a vista installation disc. ???

Thanks a lot. ;D