The link to it is here. Note that it runs in IE, not FireFox. It will:

The Secunia Software Inspector will inspect your operating system and software for insecure versions and missing security updates. A default inspection normally lasts 5-40 seconds, while a thorough inspection may take several minutes. Note: If you have anti-virus software or similar enabled, an inspection  may increase significantly in duration.

To keep your Windows PC healthy, run a Anti-Virus software (like free AVG), a firewall (I used to use free Zone Alarm, I am currently testing the built in Windows Firewall), don’t open up attachment from people you don’t now (test the attachment here if yo must), use an email service like Gmail’s or Yahoo’s that has server side anti-malware/anti-spam filtering, and scan your machine occasionally for adware, apply windows security and application security patches on a regular basis, and make frequent backups of your data.

If you get infected/compromised, you need to wipe and re-install the OS, re-load your apps and restore your data.