The way I did it is:
1. I installed the x64 Components from Vista Codecs (
click here to find proper description and download link). The reason why I used this package is to save me time manually installing Gabest splitter, mkv support in MCE/WMP and so on.
2. Manually copy the MPCVideoDec.ax filter into your system folder (32bit = System32, 64bit = SysWOW64), also do not forget to use the appropriate regsvr32 and on Vista it has to be registered with the administrative rights if UAC is enabled.
3. Make sure MPEG4 AVC is disabled in ffdshow.
4. Use the MediaControl for MCE to set the MPCVideoDec.ax as the decoder for H264 (I do not remember this process exactly as I do not use MCE - explained later)
5. You should have a DXVA fully functional
Unfortunatelly, for those ones that require subtitles, there is no option that I know of that will use DXVA with subtitles (except the MPC Home Cinema of course). This is why I use MediaPortal with MPC Home Cinema setup as an external player for movies (I do not need subtitles myself, but my girlfriend does).