The above only applies if you have one box and does not share your Internet device (modem, router etc) between two or more computers.
Of course, AVG Free, Zonealarm or Sygate is the best free software you can install and use.
If you're sharing your Internet connection, then you can also look at Smoothwall, an best-of-breed Internet firewall. The software itself is free, although you will need to install a dedicated PC for this firewall.
Dansguardian (an content filter) and ClamAV (antivirus) can be installed on Smoothwall, thus giving you better control over what sites can be blocked and ClamAV can block virus-infested downloads.
HTH
Libs