go into your bios, usually by pressing the delete key for f8 right when the computer turns on. Then in the boot device section, change it so that it boots from your drive first, save the changes and put in the windows installation cd. then when you restart your computer, it should tell you to press any key if u want to boot from the cd. then you can either try reparing your windows files (usually doesnt help) or you can just reformat.
When you get your computer back up and running, download Nod32, it is one of the best anti-virus programs. its not a piece of shit like norton or mcafee. you can either try the trial version of it or download the cracked full version from dc++ or www.packetnews.com <span class='smallblacktext'>[ Edited 25 Aug : 06:23 ]</span>