In my opinion AVG isn't just the best free one, it's the best one all around. Avast is pretty decent too.
The one catch with AVG is that the website of the creators has been hding the free version pretty deeply in the site. If you cannoy locate if (AVGFree), you can also get it from a bunch of mirror sites like majorgeeks.com
Also, if you suspect that you already have a virus on your system, you can also do a free online scan through your webbrowser by going to
http://housecall.antivirus.comNote that since this is a website, it will NOT run continuously in the background, so it will only actively protect you when you are manually going to the site and run the scan -- but it can be very helpful in cleaning up an affected machine. Also scans & removes a bunch of spyware programs, if you choose to.