If you need to know what kind of good and decent anti virus to protect your computer, AVG is an example. AVG is free downloadable for many computer users. You can upgrade your AVG to make your computer safer. But the free one is enough to avoid viruses contain your operating system.
You can download AVG Anti Virus Free Edition here or you can get it from the website www.free.avg.com if you don't trust the link.
After you install AVG, you should update the protection everyday. AVG provides every single update for free.