Action Centrer giving unnecessary messages...

This may be caused by some kind of tweaking utility 'optimising' your system - do you run any such tool?

It could also be caused if there is malware on the system.

Please download and install Malwarebytes Anti-malware (free version) from http://www.malwarebytes.org/products/malwarebytes_free/ - UNtick 'Enable free trial of MBAM PRO' at the end of the installation - and update it, then run a full scan in your main account, and Quick scans in any other user accounts.

Delete everything it finds

Also check this: Action Center - Change Message Settings

The problem is that all that would do is hide the problem - not cure it :)

Do you have other security software that perhaps is monitoring WU and alerting simply because no updates have occurred for a while? Just a theory. Often PC maker's such as Dell put in a control center that can do this sort of thing and I agree, it's annoying.
The only other thing I can think of is maybe you aren't always connected to the Internet and it is trying to poll for updates and can't? Again, just a theory.

Also check this as it may help: Turn Off Security Messages and Other System Notifications in Windows 7