I've been using this piece of software for a few years now, and regularly recommend it to my work colleagues.

Attackers gain unauthorised access to your machine by using Trojans, Worms and other malicious programs to take advantage of vulnerabilities in applications and their underlying Operating System.

The Secunia PSI is a FREE security tool designed to detect vulnerable and out-dated programs and plug-ins which expose your PC to attacks. Attacks exploiting vulnerable programs and plug-ins are rarely blocked by traditional anti-virus and are therefore increasingly "popular" among criminals.
http://secunia.com/vulnerability_scanning/personal/

In my experience, this software is more valuable to your traditional AV software (of course you should still run an AV software on your machine) because it closes the door on the attackers. Threat becomes useless if there is no vulnerability to exploit!
This is a proactive solution as against the reactive (Signature-based) AV solution.

PSI stands for Personal Software Inspector. It's free for home users, but corporate organisations wanting to use its enterprise version (CSI) have to pay.

Read more about this software on the Vendor's website (if needed), download PSI, and stay safe on the InterWeb!