Coming from personal experience, I have used Avira, Avast!, AVGfree, and Nod32 (current AV on my rig). Of all of them I like NOD32 the best.
Avira, while good at stopping the viruses from getting in, just got too annoying for me with the add that it pops up (cause it would do it randomly).
Avast!, is another good choice, but it can give you several false positives in some cases. Like if you d/l Leatrix Latency Fix.. the moment you run that Avast will tell you that something is trying to edit the registry. While this is a good thing can get annoying if you know what you are doing, and it has a habit of preventing some applications from editing the registry (again not always a bad thing).
AVGfree, is a solid choice, as the only difference between the paid version and the free is that you have to manually run the scan on your drives. While this does prevent the program from using resources that are needed elsewhere, it can be a problem if you are prone to forget that you need to run a scan.
Nod32, by far my personal favorite, I have not had a problem at all with viruses and such with this, and it doesn't interfere with performance in wow. It is also one of the highest rated AV apps out there currently.
Now, I know that there are others out there that have their own ideas as to what is best for AV, and then there are those that say that if you are smart you don't need it. But its always best to be prepared and armed then to think that you can't be affected by viruses (just my opinion). Also, if you really want to put an AV program to the test, point your browser to
Spycar.org and follow the directions. I did this with Nod32 and it stopped everything (even the removal app lol).
Sorry about the wall of text, but thought I would just add my 2¢.
