If your computer is a retail package it should have come with a "recovery cd". Pop that in and it will guide you.
If you have an OS CD you can format your C drive which will erase EVERYTHING. Most computers will ask you step by step what you want to do after a format. If you've never done it before it may take a couple of days to figure it out.
If your computer was a custom build you should be able to find software for your hard drive to make the format easy.
There is a way to format the hard drive in DOS, but I'd have to spend some time figuring it out, it's been a couple years.
My final words are.... If you want to try doing this yourself, you must anticipate things not going right. If you have a backup computer you're OK, it may take a couple days to get back up and running. The scariest part is if things go wrong. Your main resource is the internet in most cases. If you don't have a back up PC, you will have to call someone..... Who you gonna call?