Secunia offers a free online security inspection of Windows PC's. It audits a PC for insecure versions of applications using the vulnerability database of Secunia.

Feature Overview:
* Detects insecure versions of applications installed
* Verifies that all Microsoft patches are applied
* Assists you in updating your system and applications
* Runs through your browser. No installation or download is required.

Check it over at http://secunia.com/software_inspector/