Well, no one AV is going to catch everything. With that said, for the free products I recommend Avira Antivir. If anything sometimes it's too aggressive and will give false positives, but I think it's the best of the 3 big free "A's" (AVG, Avast, Avira).

http://www.free-av.com/en/trialpay_d...antivirus.html

Make sure to uninstall Avast first if you do decide to switch.