You probably have heard of it, as it is pretty well known. It is called ClamWin, and is based off of the original ClamAV that was originally a port for 'Nix systems. It has a really strong detection rate, and cleans up most anything that is out there. The one draw back to it is its lack of real-time on-access protection.
Now if you read their forums, and FAQ's they say that they are working on getting on-access scanning in on the next version which is due out later this year. In the meantime the best thing you can do is to schedule frequent scans of your files. Well, at least, that was the old school way of protecting with ClamWin.