erg i dont have a fansy all made up web page like those other dudes do so ill let them post it but heres what i would do if you are running anything but xp/2k then go like this.

Put your winblows disk in the drive and have your bios set to boot from it. Next when it boots chose the option to start windows from cd and then start command prompt.
Once you do that then type in "format c:"
hit yes and stuff
boom youve reformated
Load the windows disk again and chose start from cd and setup later.