This question I'm asked all the time, but a hard one to answer. I have a new recommended anti-virus about once a year. Most free antiviruses don't give you real time protection, meaning you're not fully protected and you have to set them up for automatic scans. Here's a few I've used.
Avira Free Antivirus – This is a strong and impressive antivirus. I used to recommend this and install this on all my clients computers. It would pick up infections that others wouldn't. I have this on my shop computer to use when I have to take a hard drive out. I still like it but the amount of popups it sends out is crazy. I leave the computer on overnight and when I turn on the monitor in the morning there is about 4 popup pages and one notification in bottom right. That is the main reason I don't install this anti-virus anymore, but it is a good one
AVG Free 2011 Antivirus– I've always liked AVG, I actually have the full version, AVG Internet Security,installed on my laptop. It doesn't slow my computer down too much. It doesn't do automatic scans of my computer, which I don't like. It's very user friendly and easy to use. I've had problems with clients installing the toolbar that comes included in the download. It changes your search suggestions and they were having problems typing addresses into the address bar. I just don't use that add-on. Overall it is a great free antivirus.
Avast Free Antivirus– Right now this is my choice as far as free antiviruses. It's strong, easy to install and setup. I've had no problems with this one at all. You will get a notification on the bottom right every once in a while but that's nothing compared to the protection you’re getting for free. This anti-virus is my pick for the best free anti-virus and is recommended this year.