AVG Free anti-virus is one of the most popular anti-virus and is widely used. It has a very simple interface and is a hassle free install and forget softare.  It’s latest update AVG 8.0 free is out now and has improved very much the interface is simpler + it now checks for spyware also. But it does not have any rootkit protection or linkscanner which you’ll get only in pro editions costing  $35 or $55 for the standard or “internet security” versions.

You can get it here.