Microsoft Security Essentials
It's very controversial that antivirus software is something that I DON'T use :). In most cases, the software is more harmful than having an actual virus. I enjoy discussing the issue but unfortunately out of scope for this post. If you must use antivirus software personally, use Microsoft's Security Essentials. It's free for home users and updates automatically via Microsoft Update. Security Essentials has recently been rated #1 by several major tech publications.