Katie,
Thanks for your response. Problems with the G4 cube previously reported were not correct (my fault, I had removed the dongle, hence demo mode - sorry). The problem with the G3 tower persists. I have tried removing the preferences file but the problem returned - demo mode seems to kick in after the computer's screen has been dimmed (even though the energy control panel has been adjusted so that the hard disc never slleps). I have now swapped over the USB dongle from a Mac that has been running OK. If the problem transfers to the 'OK' Mac, then we will have identified a dongle malfunction. Whatever the outcome, I will let you know what happens.