I was almost going to advice against AVG Free, but checking up on some recent tests it seems to have gotten back on track again, after lagging behind in recent years. That's good to hear since last I used AVG Free, though it was a few years ago, it always behaved well. (In the past 12-24 months I saw so many machines of friends and customers where really bad things had just waltzed past AVG, and haven't seen many use it since.)
Avira AntiVir Personal also get good reviews actually and consistently so for years, but it displays a nag screen about buying the program once every day. Microsoft's is in a little-league of its own in comparison to most others on both detection and update frequency. But it's better than nothing and the best of all is a careful user of course.
Two serious test sites that I'm keeping an eye on: http://www.av-comparatives.org
Edit: I'll have to recant my trust in Panda Cloud Antivirus in its current form. Security is probably great, but it has a couple of odd bugs (interface and some in some instances also performance-wise) that seem to take undue time to fix. Stay away for now regardless of its safety scores.