I have been running AVF Free for a couple of years without any problems; then again until you get infected you don't know how good/bad your AV prog. is.
I was going to give the Microsoft Security a try out until I discovered in the small print that you give MS the right to obtain some details from your PC. Things like the names of your folders...etc
Now you may have nothing to hide but my hard drive is my business and no one elses.