On the computer problems, never put more than one antivirus, they will conflict with each other and will not work correctly. Also, there are a lot of free antivirus offer online, before getting one, it is best to do some research on computer publication to confirm it is not a skam to infect your computer to make you believe you need to buy that antivirus. Keep in mind that not all antivirus will remove spywares.
If at any point you need to erase windows and reinstall it, it is really easy, specially XP or Vista, they prety much take you by the hand, just make sure you have the product key, and if you dont have it, call microsoft, they will give you one, it might take you a few headaches to get the key but it is better than buying a new windows.
If you need to manually erase a virus, you probably can find instruction on the internet on how to remove it, but if you do, be careful as if you delete the wrong file, you will have to reinstall windows
if you have to reinstall windows, try backing up all your files, programs do not need to be backed up as they can be reinstall. If you have programs that you paid for, consult with their customer service or use the following free software SWI that gives you a lot of information on almost everything in your computer (you only need the main view if you are not familiar wirh computers and it easy to navigate). Most sites allows you to download the programs and then give you the option to put your own key, keep a trail or buy it