Win 7 Guest hangs on shutdown

[Archdeacon]

Host XP SP3, Guest Windows 7 Ultimate, VirtualBox 3.2.10

Recently, after I click the Win7 Shutdown button (as I have always done), the icons disappear from the screen as normal but I am getting no Logging Off... or
Shutting Down... and the system hangs indefinitely. The only way out is to use Close and then Power Off. A new start then wants safe mode (which eventually shuts down OK), but starting normally works fine until trying to shutdown again!
Recently I have noticed that the Sound Service (Icon) is crossed out sometimes, although sound is working.
What am I doing wrong? There has been nothing installed, third party drivers, etc, recently.