There is another free AV program called
Threatfire - It runs along with your regular AV but it protects in a different way.
Ive been using it for 4 months now along with Zone Alarm.

[How can ThreatFire protect me when traditional antivirus can't?

ThreatFire continually protects your PC against attacks by detecting malicious behavior, such as capturing your keystrokes or stealing your data, instead of only looking for known threats like normal antivirus software. By implementing sophisticated real-time behavioral analysis ThreatFire is able to stop never- before-seen "zero-day" threats solely by detecting their malicious activity.
Zero-day threats are usually designed to take advantage of new vulnerabilities or exploits that are currently unprotected by traditional security products. They are usually distributed in huge quantities very quickly by mass email (SPAM), website hijacks, instant messaging or over peer-to-peer networks. Because they are undetectable they are able to wreak havoc and compromise your PC even when you have up-to-date antivirus software installed.

ThreatFire's patent-pending ActiveDefense technology offers protection against all types of internet threats - both known and unknown - spyware, adware, keyloggers, viruses, worms, Trojans, rootkits, buffer overflows, and other malware. ThreatFire uses its unparalleled protection to hunt down and paralyze those threats that are either too new or too clever to be recognized by traditional "signature-based" antivirus software.]

more info-
http://www.threatfire.com/