Either back up data or accept that its gone, then put in the windows cd much like when you first installed windows. Vista walks you through it.
All the easier ways feature backing up a working operating system first then restoring from it, since you dont have a known working operating system this option is closed to you.
Personally I use an XP cd which I edited a bit with a program called nlite, the vista version is called vlite. It installs drivers for me, the missus laptop, her sisters and her mothers, along with programs like vlc, avast and firefox. This took me a good few hours to set up but it saves an enormous amount of time once done. For just one computer, keeping it offline throughout, install windows, then all your programs and drivers, defragment and move folders around as you like. Then image it with whatever approach you like, later on when it's all broken you just copy the image back across.
I zero the remainder of the partition then use dd piped through gzip from an ubuntu live cd. There are friendlier ways though, I think Norton ghost is a favorite