Computer help please

This message has started appearing on one of my PC’s. Every time I go on the net up it comes and shuts the system down. It gets very annoying :mad:. I have two PC’s both with legitimate versions of XP SP1, they are both networked together and have been for about six months, I have looked in the Services and can find nothing to stop it. The message does appear in the Event Log but says it doesn’t know the cause. Anyone got any Ideas as to how I can stop it?

Desperately needing your help, Plazz