Avast http://www.avast.com/eng/avast_4_home.html offers it’s state of the art anti-virus software, free to home users and for non-commercial use. 

“avast! 4 Home Edition is a full-featured antivirus package designed exclusively for home users and non-commercial use. Both of these conditions should be met! Our company offers the Home Edition free of charge, since, in our opinion, it is possible to avoid global virus spreading by efficient prevention; however, many user are not able to or do not want to pay for antivirus software.”

More on the site, check it out.