Well, on XP, when you start the installation process, you get the chance to either 'quick format' (erases header info, iirc) or a full format. That will do it. If you really want to go over the top, you can download a boot disk (assuming you don't have one) and run fdisk from the command prompt (without launching windows). From there you can do a low-level format.
IME, doing the full format in installation does plenty.
I'm not a computer geek, so take this for what it's worth...