The computer that had rebooted itself on Friday was fine all day and then the second I turned the monitor off yesterday – it did it again. Turning off your monitor really shouldn’t reboot your machine should it?!
It was then I realised that plugged into the USB hub in the monitor was my Sharp Zaurus docking station, and the mystery was solved. Zaurus plugged into the monitor hub and turned on, computer reboots when monitor turned off. Zaurus either not plugged in or turned off, everything is fine. I still don’t understand why this should have been happening, however I’m glad to have found the cause – the Zaurus is now plugged back into the USB on the computer and I can turn my monitor off without killing my machine.