You don't.
You may format the HD, but this will wipe all the information on it, including the OS (windows) so you may want to think hard about doing this.
Done thinking?
Ok, open a DOS window (Start/Run - type 'cmd' without the quotes, a little black window should pop up on your screen)
On the little black magic window type the following, without the quotes: "format c:/ -s" and press enter.
It will ask you to confirm. Answer "y" and press enter.
a few minutes later you will have a freshly formatted Hard Drive and an unusable computer.
Alternatively you may want to boot from the original Windows CD and select new install, and answer Yes when asked if you want to format the drive.
Same endgame, all information gone, HD reformatted, but at least you have a working OS.
Good luck,
Bob Smith
January 2009