Have you maybe installed a new driver, some says the driver needs to be uninstalled before installing a new one...
Have you tried to vacuum the pc? if alot of dusts in the pc, can cause the overheat... But I cannot tell why the cpu goes 100%
I also had a PC that have been got bad cause of Windows Updates, if choosen all... Some of the updates also can cause problems...
if you have a System Restore before the Incident of the problem, try that also