I myself avoid the free antiviruses. I used nod32 for a long time now, recently I've tried Webroot SecureAnywhere, and gotta say, HUGE fan. Probably the most minimalistic system resource use antivirus out there once they do tests on it again.
But anyway, that doesn't answer your question. If I have to install antivirus on somebody elses unit and they won't want to pay, I go for AVAST. Benchmarks have shown it uses least system resources out of all free ones.