This is for anyone who might have installed Windows Defender Beta and suddenly has started getting either an error message, or found that it is not installing that security update that keeps coming up.
What the program is trying to tell you (rather badly I might add) is that Windows Defender Beta expired on 31st December 2006.
To solve the problem, you need to remove Windows Defender using the Add/Remove programs option in your Control Panel, and download and install the new version. The Windows Defender homepage is here. Of course, you have to go through the validation process but otherwise the program is free.