Ok Henri,
It seems like we tried all the simple steps. Try running a
sfc /scannow command to see if it can find and fix the problem. You may need to manually start Windows Sidebar afterwards.
How to Repair and Verify the Integrity of Vista System Files with System File Checker
If that does not work, then try a Repair install. This will allow you to repair your Vista installation without losing your files, settings, and programs.
How To Perform a Repair Installation For Vista
Shawn