Unable to logoff on an intermittent basis

I am unable to logoff (if not shut down) on an intermittend basis. I can
continue using the currently running account (limited user of administrator)
but Windows XP SP2 does just not logoff (or shut down). How can I determine
the root cause?

Please note that my antivirus / firewall / spyware / parental guidance
service is based on Radialpoint/PCGuard technology.
--
Dieter (UK)

Basically this is most often due to a program or service which refuses to
close when sent the shurtdown signal.

As a workaround you might like to try:

http://users.pandora.be/jbosman/poweroff/poweroff.htm

which I've found to be useful when I want to make sure a remote server
reboots properly and doesn't 'stick' on something.