For comparison purposes, I ran AVG free and wife ran Avast for about a year. Neither of us caught any sort of viral infection. We do tend to both practice "safe computing", so YMMV.
We have since dropped the free packages in favor of Sunbelt Software's Vipre. This program is very reasonable in cost (annual subscription), can be run on up-to-five computers (we have 4) on the "family" license, and has totally blocked all mal-ware from our computers. It has a very small memory footprint, and lower CPU usage than either Avast or AVG.