I've had this in the past - I believe it was due to either malware or a corrupt registry (or both!)
I managed to solve it by re-activating Windows using the 25-digit Microsoft activation number - do you have this? It should be on the System page of Control Panel, or can be extracted using Belarc Advisor if not. Once you have it you need to type slui.exe in the Run box and run this, entering the code when prompted.
Worked for me anyway, unless anyone has any other advice, hope this helps...