I ran an image backup program(Drive Snapshot)over night for a large amount of data (300GB on a 500GB HD) switched off the monitor (might be relevant) and went to bed. got up twice and checked both time the program had been paused and restarted a few seconds after I moved the mouse...

Istar - Check your power settings. If you've got Windows set to go to sleep or standby after a given period of inactivity, it may do so - regardless of if a task is running. Make sure you're on the High performance scheme.