Microsoft Security Essentials.
Apparently I'm not stupid enough to download a virus, so I don't need some bulky paid (or cracked) software.
Additionally, all other free AVs are full of nags, ads or both. They don't provide anything I need that MSE doesn't have and, to be honest, I only have MSE so people don't say "wtf you dont use an av? whats wrong with you?!".