Hi
The standalone codec from MPC HC X86 does hardware acceleration on my ATI HD3200 with Mediaportal and XP Pro SP2
Goto Media Player Classic Home Cinema External Filters for X86 (XP)
http://sourceforge.net/project/showfiles.php?group_id=170561&package_id=264678
Download
mpchc_x86_v1.1.604.0_MPCVideoDec.zip
Save the file in C:\Windows\System32
Click Start -> Run
Then type
regsvr32 MPCVideoDec.ax
You should get the message that the file is registered
Then open Mediaportal Configuration
Goto Television and change the H264 codec to MPCVideoDec
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If you have problems you can type
regsvr32 -u MPCVideoDec.ax
To unregister the codec
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EDIT: THE ABOVE DOESN'T WORK - SEE POST BELOW, PERHAPS MEDIAPORTAL WAS STILL USING PDVD8 DESPITE HAVING MPCVIDEODEC SELECTED UNDER H264 (AS SUGGESTED BY MCRAENZ)