ZoneAlarm and Microsoft Security Essentials are probably the best free security suites on the market.
ZoneAlarm is pretty lightweight, has some of the best detection rates (up there with the paid ones) with fairly few false positives, one of the most capable when it comes to removing infections, is pretty user friendly and doesn't nag too much.
MSE is probably the most light weight, easiest to use, least nagging and has the lowest false positives (though they're still there, if infrequent) of any security suite I've used, it's pretty good at finding and removing threats but certainly isn't the best.
I also hear a lot of good things about Avast! but I've not seen it much in action. I hesitate to recommend it since I often see it running on infected computers while it sits in the background doing little but asking people to pay for it, after which I get rid of it (or try to - it sometimes doesn't like to uninstall) and install ZoneAlarm then everything it sweet. I haven't run into ZoneAlarm or MSE in that same scenario as of yet.
Malwarebytes is also pretty good for detecting and removing infections however last I checked it doesn't offer real-time protection in the free version.