i'm a big supporter of panda cloud
in our information technology class that's the free av we install on all systems that come through
it is pretty light on the system and scans everything that comes through, comparing it to a cloud-stored list of known fraudulent software and viruses and axing processes that can compromise the system, as well as using heuristics to see if something is potentially harmful but not yet known
it auto-updates and you can also initiate a deep scan with it; only gripe is it doesn't have as many features as some other avs, but for what it does have it's excellent for being free