The Secunia PSI is a free security tool designed to detect vulnerable and out-dated PC programs.
Update: Unfortunately version 3 is not very useful and a huge resource hog so I have dropped this tool.
I have been using Secunia Personal Software Inspector (PSI) for some time but was reluctant to recommend it as it was a bit difficult to use and was a bit of a resource hog. However they have just updated it to version 2.0 and it is a significant improvement.
Firstly it is now much easier to use and correct any issues it finds.