Thats good, you could always add a firewall program, hardware firewall, or just stick with the default one. Another extra would be using google chrome or firefox as your web browser and for 32 bit systems you could use sandboxie.
I would go a step further than scanning regularly. Run an active scanner. FYI though - the latest trojans will fly past avast and avg free versions (don't know about the paid one's) - seen it happen many times.
Anti-Virus programs are so cheap these days why not spend the $30-40 per year on a program like Eset Smart Security - bundled software firewall and anti-virus (with great heuristic scanning - meaning it should detect most new viruses in the wild before they are studied and defined) You wouldn't regret it - and ESET is very lightweight even with the Firewall.