Another thought based on past experiance, do you have a network card installed, even if its not connected, if its got any drivers going for it, it does like to check its connection every so often which ends up as an interrupt and a restart of your defrag. The way around it is to disable the network card in the control panel - system - devices
what would help is a complete system spec and anything that you have changed between the working and notworking, I think we all guessing in all sorts of directions without the basic info.
Rickity