Do this now.

Secunia's Software Inspector is a browser-based Java application that scans your machine for vulnerable software based on their vulnerability info. Before I ran this, I thought all my bases were covered, but now, heh, guess again.

Edited to add: older versions of the Macromedia Flash Player can be very difficult to remove. They are not removed automatically when a new version of the Flash Player is installed, leaving your system potentially open to the vulnerabilities in the older versions. You might find this link of use - you actually have to download this uninstaller from the Macromedia site and run it to uninstall Flash. Talk about easy removal...

For one of the old Flash versions I had, not even this worked, so I eventually removed it by manually deleting the file.