I heard idiots rave about Essentials. I used Avira which works ok, but is not that impressive and you get a nag screen. It is free though. Well, within a week of Essentials I had two infections. I won't bother with it again. Back to Avira which I like because it runs during gaming without bothering anything. I had Avast at one time but on MY system would not run and game at the same time. I am not one to turn off the antivirus programs to game.
What I will tell you is absolutely top notch and free. It is Kasperskys VIRUS REMOVAL TOOL it is NOT an anti virus. You download it and install it, run it, it cleans your system squeaky clean and then you uninstall it. It is a somewhat large download, like 110meg. But it is the best removal tool I have found. So, I compromise and run not the best antivirus and once in a while I run the virus removal tool. Then what Mott says. You are the best defense against viruses just by your actions. Oh, and if you run a MS operating system that has UAC? Use it. It does its job and helps out too.