At work (a university) the central IT has chosen to license Sophos. It is, well, crap to put it mildly and takes up amazing amounts of resources. So, instead we use Security Essentials on many systems. Works well, it has successfully stopped viruses that users have tried to get. Pretty light on resources over all, not the lightest weight program I've seen but up there.
Best one for free I've seen. Personally ESET NOD32 is my favourite and what I license for home, but if the price requirement is $0, then MSE is what I use.