I tried out Avast4 when I read that AVG were going to stop their free version (this may have been a false rumour, as I can see no sign of it happening, as yet)
At first I liked it, Has some natty features & skins, but I found that it used much more resources than AVG, plus at around the time of the RSPCC.exe (for XP) scare, I discovered that Avast calls up RSPCC.exe and this was constantly trying to access the internet. Their Tech support -on the forum-assured me & others who were concerned that this was normal activity. But it was supposed to be only communicating internally, not over the internet. I dropped it and have been back with AVG ever since-no regrets-much simpler, and fewer resources used.
How good it is at virus detecting I have no idea though believe it gives more false positives than AVG.