Quote Originally Posted by Martin View Post

As well as checking the various BIOS settings, try some empirical tests...

Turn the mains OFF and then ON to a "switched off" (one that has the problem) PC to see it it wakes up.
Switch a kettle on (and off) on a socket near to one of the problem PCs - we had one in the office that turned on in the mornings as the kettle boiled!

Well we tried that and no joy.
Trouble is that thye only seem to do it on either a Friday or Saturday night!
Ah well we shall have to wait and see.