In addition to antivirus software, a good personal firewall is also a great addition to your computer's list of protection tools. If you're on a network you ought to have a firewall - even if you have a hardware firewall between you and the internet.
Why? Because other computers on your network might get a virus and, being on the same side of the firewall as you, will not be inhibited by the hardware firewall. In addition, a guest on you rnetwork or someone 'war driving' might come across an open wireless on your network and gain access.
As with antivirus software, there are some good products that are free. You can find them at www.download.com: Zonealarm, PC Tools Antivirus, Comodo Internet Security, Ashampoo firewall and others. Zonealarm is still my favorite at the time of this writing, but it's beginning to get a bit bloated and I'm looking at others.
Still, a good firewall, like antivirus software, is good insurance.