Here's the funny thing about AV software;

Many users assume that because their AV software is not providing alerts to them about infections that their system is clean. This is absolutely 100% incorrect and a rather dangerous point of view to take. The lack of alerting does NOT equate to a lack of infection.

There are three primary factors I look for in "AV" solutions, the first of which is functionality far beyond simple "AV".

1) Rapid releases of signature based detections - covers known malware and known bad actors.
2) Reputation based engines - analysis of software based on the source of the software, popularity, and validation of hashes.
3) Integrated system protection - provides protection against system level changes not tied to a specific application but appear rogue.

Right now I'm running Norton Internet Security 2012 to satisfy those needs. The system overhead is quite low and you can see at any point exactly how much of your clock cycles and memory are being used by the software and how it effects the performance on your machine. Speaking of performance; one of the things it's great at doing is establishing baselines for what processes SHOULD be using and then providing alerts if something starts going crazy. I pay about $80/year for protection on three machines. I plan on sticking with NIS until something else comes along that does a better job more efficiently.

FWIW: Most people are infected with some sort of malware/adware and simply don't realize it or don't detect it.