Avast! and AVG 2013. And Microsoft Security Essentials. I've used all three, and I don't really care too much about which one's better, as they're all suitable to me. My true champion, my defender of my personal computer, however, is Malwarebytes' Anti-Malware. If nothing else, you need that. For me it's more of a question of removing malware once it's inside than preventing it altogether.