Comp won't shut down

Start menu->shut down, programs and everything stop running, desktop disappears, the little 'saving your settings, windows is shutting down' box shows up, that disappears, then nothing.  It just hangs there.  I can ctrl-alt-del, and it still won't shut down.

Firefox always shuts down when requested.  Furthermore, it can restore it's previous state, even if it crashes!

www.getfirefox.com