Can't log off computer

For a few weeks back I begun having difficulties logging off my computer, it stuck in the "Shutting down" screen. Then after a few days, it shut down properly again, but now the issue is back, and I can no longer shut my computer off unless I use Safety Mode.
I've looked at the processes running as I turn it off, and only logonui.exe and explorer.exe are using resources as I log off, and I assume that is normal, usually about 10% of the cpu capacity.

I have scanned my computer thoroughly for malware and viruses, but detected nothing.

Any idea what the problem might be?

 

i have had problems with yahoo messenger, and adobe reader (some sort of adobe loader prog) . if you have those, see if disabling any of those or ending the process(es) helps.

 

I have never had Yahoo Messenger installed, and I closed a Adobe process that was running, but that did not help, so the error must be something else.