One very useful soft: Secunia Personal Software Inspector (PSI)

Out and Free! Just as stated on the banner, this is one piece of software that you should NOT be without

VULNERABLE ?
Did you know that many of the hacker attacks and security threats today exploit software vulnerabilities and code flaws?
UPDATED ?
Keeping your PC and especially your 3rd party programs updated requires you to search the Internet for updates and patches on a regular basis - for all the programs installed on your PC.
WHATS ON YOUR PC ?
The typical user has 30-60 programs installed - do you know which programs you have installed? Do you know which programs expose you to security threats?
SECURE ?
Is your PC secure? Do you have all the latest security updates and patches?
PROTECT YOURSELF !
Security patches are usually free and available for download from the program vendors. Let the Secunia PSI pinpoint exactly which patches you need to secure your PC.
The Secunia PSI is a free security tool designed with the sole purpose of helping you secure your computer against vulnerabilities in programs.

(Thx Eric for the link)