first things first, make sure you're prepared to do it
if you've never done it before, have at LEAST the following:
your OS installation CDROM (ie Windows XP, make sure you've got your CDkey+install CDROM)
another PC with an internet connection (fast if possible) and a way to transfer files to the formatted PC (burn a CD or use a USB drive) This is for when you need to update drivers right after installation, before you can even set up your internet connection.

now since I'm assuming you're using windowsXP pro, check this out: http://www.tweakguides.com/XPTC.html

and I recommend printing the whole thing out, its a good reference to have. read it carefully, make sure you understand it before you try it! I helped my friend reformat his pc on saturday, it was over and done with in aboot 4 hours and we played WoW for another 6. if you've got questions I'm on msn [email protected]