How to format the hard disk.

See Hard Drive FAQ.

If you have to reformat a hard drive, and you're certain that you want to do that, try googling "Clean Install Windows":

Let me google that for you

There are lots of sites that will give you a lot of information. If it's Windows XP, the simplest description is to 1) boot from the CD. 2) delete the current partition and format it, and 3) reinstall Windows. This is all done from the Windows CD install disk.

If you don't have a Windows CD install disk -- many systems come with a disk image on a second disk partition -- you'll probably have to restore an "image" to the disk. This may be done by hitting something like <F10>, <F11>, or <F12> on bootup and using the utility that is installed. Alternately, you may have to reimage from a set of CDs/DVDs. The instructions vary for these systems, but the operation effectively reformats your hard drive.