This morning when I got to my Apple Mac Pro, I saw that VMWare Fusion, which had been running Window XP, was stuck at Shutting Down Windows. Presumably Windows decided to do an upgrade during the night and got stuck when it tried to reboot.
Going to the menu 'Virtual Machine->Shut Down Guest' didn't do anything. The trick was to hold down the option key, which changes the Shut Down Guest to 'Power Off'. Selecting this worked fine.
Hope this tip helps someone else.