I'd stay away from AVG. It's detection rates are lackluster, and last time I used it, it had some irritating update behavior. I've never used Avast, but it's seems alright. It's detection rates are bit lower than Avira, but I think it comes down to which annoyance you prefer; registering for Avast, or dealing with an update popup from Avira. The popup can be disabled, but it doesn't bother me. I haven't bothered fooling with it on this machine.
I had some problems with MSE, and uninstalled it after a week. Early reports have indicated good detection rates, but avcomparatives hasn't released detection rates for it yet. It has a simple interface, perhaps a bit too simple for my tastes, and is intuitive to use. I found memory use to be high, and it crashed Explorer when trying to run VLC setup. I also had some web slowdown, but I didn't test it, so I can't blame MSE. It could have just been coincidence.