1. do you have a Vista CD and a legit CD key?
if so...restart your computer, setting boot priority with CD-ROM set to boot first, if you already dont have it that way...
if so...restart copmuter with OS Disc in your drive, boot from disc when it asks you to...then when you get to the installation window, press install...format your hardrive from any partitions you may have, create a new partition, and click fresh install...and you should be good to go afterwards.