Hmm, there has been problem with amd and windows 8 lately. Some people's system don't pick their card up or have problem with games running on really low fps. I think the cause for this is because the gpus now are based on windows 7 and the drivers to adapt them to windows 8 aren't the best. Best thing is to check you have all drivers updated, restart your system, etc. and look on windows 8 forums about this problem. It's a new OS so there's bound to be bugs.