Can anyone confirm that Windows Media Player 12 under Windows 7 uses the GPU for HD playback? My 1080p files play smoothly with only 5% CPU usage while under Windows Vista I have between 50% and 60% CPU usage. Video card is a Radeon HD 3850 with the latest Catalyst 9.3 drivers.
Sounds like you haven't configured Vista to use DXVA.
There are so many settings that can effect this. Dxdiag, WMP11, ATI control panel.
Or perhaps your using an alternative 3rd party codec on Vista that doesn't support hardware acceleration. Then again, Windows 7 does support a few new codecs OTB.
Your driver is fine though.
What format is your 1080p content in?