Microsoft is now offering it's free Anti-Virus offering, Microsoft Security Essentials at http://www.microsoft.com/security_essentials/default.aspx
Iobit is offering a free license of IObit Security 360™ PRO at http://db.iobit.com/license-free/win7-special-offer.php
AVG Antivirus free version 9 at http://download.cnet.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html?tag=mncol;txt