During the install of Windows one option is to format the hard drives.
So you put the Windows CD in, "boot" from the CD, begin the install process, then when you get asked do you want to format say "yes".
Aftet the format you can then cancel or end the Windows install.
Why do you want to do this, are you trying to sell the PC and you are clearing the drive, or are you going to put another operating system on it?
You realise you will lose EVERTHING off the C drive (personal files and so on)