I have an even weirder one.
My CPU monitors say 100% for Core1, bounces around between 50 and 100% on Core2... but when I check Task Manager, there isn't a single process above 5%. The usage certainly doesn't total even 30%.
I've disabled drivers
for devices that are not in use currently (SATA PCI card, ATI video capture card, and all USB card readers).
I've disabled indexing, which took an incredibly long time.
I've updated all drivers for existing hard drives and the controllers.
I'm still at 100%, often on both Cores.