I have seen shutdown scripts cause this behavior or similar behavior, and I've also seen anti-virus programs cause this behavior. It's almost like Windows Explorer is waiting for the AV client to finish, and the AV client is waiting to get exclusive access to some file Explorer is using. Once or twice I've seen similar kinds of things on laptops. The button control widgets don't shut down cleanly, and cause Explorer to wait for something which never happens. I've also seen the HP DeskJet monitoring application (or similar software) cause all kinds of weird issues. Indeed, if you're using software-based hardware monitoring and control programs of any kind, including virtual CD drives, it could also cause something like this.
You say you've seen the same behavior on 5 or 6 PCs. What's the same about these PCs? Same manufacturer? Same AV client? Same printer or camera software running? Have they all had Nero? Do they all have ATI display adapters and you run the ATI Catalyst Control Center? What have you had installed on all six or so PCs? Is it just shutting down, or is it also when the systems restart, go to sleep, or hibernate?