AVG Free is probably the best free anti-virus software out there.  It does its job by protecting you from viruses and spywares.  Unlike other anti-virus, you don’t need a subscription every year to stay up-to-date with your virus definitions.  AVG will automatically search for updates everytime you turn your computer on and will do daily scans on your PC (this option can be modified).  I highly recommended anyone with a computer without an anti-virus to download AVG Free Anti-Virus freeware.  You can download it at http://www.download.com search “free avg”

AVG Screenshot