I have used a couple of the paid programs and had no real trouble with them. Not long ago I started checking out the free AV programs. I tried Avast Internet Security's Version 7 free trial, it did OK as well, it had a lot of "neat" features to it but I soon found out I did not really need or use a lot of the features it had, so I downgraded to Avast Free version 7 and ran it for a while. I had no problems with either Avast other than version 7 Free seemed to slightly slow down my email and web browser. I got invited to try the Beta version of Avast 8 and I hated it. It slowed me down a lot and it had a lot of bugs that needed to be worked out. Avast version 8 really bogged down my email for some reason and I did not like that at all. It was a huge mistake for me to try the Beta version and I had trouble removing it from my computer.
I read up on MSE 4.0 and wanted to try it. MS has come a long way with MSE 4.0. When I finally was able to run the Avast uninstaller tool it took out some important Microsoft files from my system too. No matter how I tried I was unable to install MSE. I began to wonder if the MSE files were missing then what else was missing too? So I saved all my important files on a flash drive and used the HP recovery tool to reset my computer back to its original factory specs. I spent several days setting the computer up the way I wanted it. It really was not a bad thing to do, I got rid of a lot of programs and files I no longer needed.
I am now running MSE 4.0, have been using it for about 2 months with no problems at all. My computer is very fast and efficient again and I do like the fact that MSE is made by MS to integrate with my Windows 7 OS. I usually update it twice a day myself but I have found that its scheduled daily update works just fine by itself. I use the paid version of MWB Pro and have set up MSE and MWB with exclusions from each other and have had no problems at all with either one. I occasionally use the free version of SAS to get rid of unwanted cookies.
I have learned that keeping my operating system and all of my programs updated is critical to keeping malware off my computer. I use Secunia to monitor my system and alert me to any necessary updates. I also use Firefox version 19.0.2 with a few add-ons like Web Of Trust, Ad Block Plus, Web Mail Ad Blocker, Flagfox and Key Scrambler to help keep malware at bay and help keep me from clicking on a bad site or link. I have tried several AV programs and I believe you can get good protection from free programs. Using common sense when surfing online and staying away from known malware infested sites also goes a very long way towards keeping my system clean. I have learned that no AV program, whether paid or free will catch and/or remove all malware, but I like MSE and will stay with it. It is light and simple and works great, does its job very well without any problems.