I don't know if it's been mentioned in this thread, yet, but I've been using Panda antivirus for a while. It's completely free, seems to have a very small operational footprint, and has generally scored well in most AV testing. I should say, though, that our risk for viruses is very low. Our web surfing is minimal, we don't visit questionable websites, and we're careful with email.
I've been using computers regularly since the late 1980s, and I don't remember having a virus. Ever.