There are quite a lot of freeware antivirus software out there, but I guess those that worth trying are the ones like AVG anti virus, Avira AntiVir personal edition, microsoft security essentials, Avast.
As the saying goes, nothing is free. Either there are limitation to the features or you will get to see a lot of advertising built in the program. If you don't mind the advertising and and don't want to spend money, then I suggest you go ahead and try one of these. They are just fine.
The one I'm using both personally and in my work environment is NOD32. This is just my preference based on my years of experience, and nothing beats NOD32 and doesn't cluttered your system and run silently in the background nor does hype like Norton, McAfee.
I'm testing other few anti-virus software and only select a few good ones that is available in the market. But once I get the results, I'll put up some statistics and recommendation on my website.
Hope it helps.