Damn, my knowledge of AMD and Windows 8 is limited. I know on my intel I can limit the amount of active cores in BIOS. I would start your search there.
I did install Windows 8.1 on my new build, kept it for a week. Reinstalled Windows 7, oh the joy of going back to Windows 7 after trying to work around Windows 8 seriously unfriendly interface. I thought all the talk on the internet was everyones usual reluctance to accept change but no they are pretty much all correct... it is a step backwards.