I use avast! It's free.
I don't like the interface very much though, the people who built this went out of the way to make it look cute and futuristic, lots of bells and whistles when you click stuff. It gets very annoying. So there might be something simpler out there. Since I (purposefully) don't use the interface that much, it sort of stays out of the way. I just let the protection services run and do a boot scan every couple of months. That's my idea of virus and malware protection, anyway.