With Linux, keeping up to date with patches and updates is much more important than anti-virus. Because Linux is used mostly on servers with no actual user, most of the security concerns in Linux are from a software vulnerability allowing an intruder to access your system, rather than a virus accidentally run by a desktop user. The updates will fix vulnerabilities as they are discovered. (Granted for home use, your router's firewall should block most intrusions. Be sure to update your router's firmware regularly - most routers are just tiny computers running Linux.)
Ubuntu should be configured by default to auto-update. But you should verify that it is working (sometimes the correct repository addresses do not get entered properly by the install script).