Sorry for confusion. The best way to avoid viruses is to format your hard drive and install Windows onto clean HD. There are 2 problems with this approach. You will lose all your data (unless you can back it up). And you will need a bootable CD/DVD with Windows (you can also use USB drives,... for that).

If you have a clean disk, you can install Windows onto it as well. IMHO it is the second best approach. You will also need a bootable CD/DVD.

Just don't make another installation of Windows from your current version.

Antiviruses can't give a 100% protection against viruses. Some of them, especially new ones, can't be detected.

p.s. If you have friends who know more about it, it would be a good idea to use their help.