My Computer Will not shut down

Hi All,
Can anyone give me any clues as to how I can solve this very simple problem.
I have to click on "Turn Off Computer" several times before my computer shuts down I don't know if it is a coincidence, but it seems to have started doing this since I installed SP2.
Please help if you can as I have sometimes unknowingly, left my computer switched on all night, having beleived it had shut down.
Cheers.

 

First thing I would check is your mouse! switch it with another (sometimes we get luckey with the simple things)

 

Stray infra red and/or Radio waves?

Another long shot? Is your keyboard and/or mouse wireless? I have seen strange things happen when electronic stray waves play havoc on components that they are not meant for. My TV would turn off O.K., but then turn on in the middle of the night when my neighbour opened his garage door. I used tin foil temporally until I tracked down the problem.