I don't personally use an Anti-Virus application, except for on-demand scanning. I prefer to just run Windows as a limited user (when I have to use Windows at all, I am primarily a Linux user), and then just grant administrative permissions to applications as I use them. If something I don't expect to need administrator permissons requires it, I simply don't allow it. Combine that with the necessary bi-yearly format Windows needs and you should be good to go.
That said, if you must use an Anti-Virus, I have used both the free versions of AVG and Avast and still install them on computers I repair for others, and they work just about as good as any other AV.