Greetings,
I'd recommend
Avira.
It's free and has a better detection rating then AVG - even better than many commercial products (see
AV Comparatives - click on the "Comparatives" link to the left and then the latest available "Online results").
I switched to it from NAV 2000 many years ago, due to the latter being a resource hog.
It's light on resources since it's designed to scan processes as they start up. The paid-for version scans emails - the free one does not, but then it doesn't need to do so, as anyone opening a attachment essentially is running a process, which Avira then scans and catches if it's malicious.
It also has rootkit detection ability and heuristics for malicious behaviour.
I've never been infected with a virus - granted, as someone in the computer industry and who volunteers on security websites, I use quite a few anti-malware applications, many of them free.
Kindest regards,
Dragan Glas