From the way you ask your question, I get the impression, that you are not familiar with formating your pc and install windows xp or vista. First of all you need to get your self a legal copy of Win xp home, pro or vista.
If I were you, I would try to get ride of your viruses.because when you format your pc, you loose everything, including your personal and important files. like documents, music, video's et etc. You will have to make a backup of all those files Either burn them on a dvd or copy them to your second partition d drive.
To explain how to format now and install windows, it too much to do in this answer. I would find a friend or somebody who knows what he is doing. Otherwise I see big and I mean
BIG problems.
Instead of spending 100 dollars on the windows cd. why not get yourself a good internet security program to get rid of all your viruses
I tried nearly all of them and found Trend Micro Internet Security 2008 the best.
Why not spend some money in getting something like that
http://us.trendmicro.com/us/products/personal/internet-security-2008/