Windows XP Pro Not Shutting Down

> When I try to shutdown, I get the window with the option
> to Standby/Turn off/Restart but when I click Turn Off,
> the system goes right back to my Windows Desktop. It is
> the same for Restart.
>
> The only way I can shutdown is to Log Off, then Switch
> Users, and click Turn Off Computer at the bottom left
> hand corner. The Standby/Turn off/Restart windows comes
> up again and if I click Turn off, a window pops up asking
> if I'm sure I want to shut down, other users may be
> connected. If I click OK, it shuts down fine.
>
> This is a lengthy process and I would like to know if
> anyone else has the same issue and if there is a fix or
> any idea of how to rectify the situation.
>
> Thanks!