I did use AVG for years and it was quite good, but it screwed up a couple of things on Vista when that came out. Then I switched to Avast and it was better, I thought. It has one really neat feature. You can set it to do a 'boot time scan.' To do that you have to restart the computer. It stops the restart about half way through and does a complete scan while nothing else is moving.
The one time I did have a serious trojan that boot time scan got rid of it completely. I don't use Avast any more, but if I do get into trouble, I'll download it again just to be able to do that type of scan.
The one I use now is Microsoft Security Essentials, and I think it is the best. It is a free download. It is sort of fashionable to bash Microsoft these days but they do some really good stuff too. They have been keeping track of pretty much every virus and attack system on the net, and MSE makes use of the info.
I read a lot of website and computer forums and I have never seen a negative comment about MSE. (But go ahead and be the first if you have a problem with it
It does have one minor quirk that I don't like, but almost every program I've ever used has something I'd like to change.