Also in Windows 8 and Windows Server 2012 if the Hyper-V is installed and hpyervisor is running, hardware acceleration is not working. Neither in flash player or in any media player (wmp, wmc, vlc, mpc-hc, etc) dxva.
I have HD4850 http://www.gigabyte.com/products/product-page.aspx?pid=2856#ov I tried it with 12.6 legacy for Windows 8 and 11.9
The internet is full with this problem:
This problem is not present on Nvidia cards. Please fix this issue with a driver update or any other way.