One of the most controversial changes in Windows Vista is the redesign of the way in which you can shut down your computer. The Log Off and Shut Down buttons are no longer in the same position even if the new buttons in Windows Vista may give you the impression that they have the same functionality.

So... where did the Shut Down and Log Off buttons go?
If you take a closer look, you will see that near those two big buttons there is a small “arrow like” button. If you press on it, a menu with 7 options will appear. Log Off is the second from the top and Shut Down is the last.
Vista Shutdown button


Source Site: Windows Vista for Beginners - Windows Vista tutorials
Source Article: Where did the Shut Down & Log Off buttons go?