I use AVG. I did try Avast for a bit. I liked it. It has nice features, required low RAM ( MS recommends 1 GB ). The only problem I ran into was in watching streaming videos. It would be choppy. If I turned off the Avast shield, it was fine. Adding the sites ( such as youtube ) to the exceptions reduced the choppiness by about 90%, but it was still there. It was too bad since I really liked the features.
Haven't had any problems with video streaming on my desktop or laptop. The netbook, on the other hand, struggles a great deal but that is probably due to having a weak CPU and no (I don't think) dedicated video processor. Maybe that would be a good candidate for the MS program instead of Avast.
I'm going to keep 'em both for now - experiment with the relative features of each to see if I can keep them co-operating on my machines.