Can't shutdown

[Jim Byrnes]

I am using Ubuntu 10.04 and seem to be stuck in an endless loop where I 
can't shutdown the computer.  I click on the shutdown symbol in the top 
tray and select shutdown from the drop down menu.  I get the shutdown 
confirmation dialog and select OK and the screen blanks.  Then a dialog 
appears asking if I want to log in as my username or other.  I then end 
up back on the desktop.

How can I correct this so I don't have to simply use the power switch?

Thanks,  Jim