This win7 system - a service "avgsrv??" was causing the processor to hit 100% for hours, maybe I've got some wierd data on a back up drive it doesn't like.
You've not got some kind of scheduled scan set have you....? That kind of thing's caught me out enough times before.
Probably was, but it's uninstalled now so I can't tell, even so it should have run at a lower priority - I can accept 100% CPU usage after all why have we got them, but it should back away when I need to do something. It was the trashing of my ethernet card
I was most surprised by.