I haven't seen anything about this, so I figured I'd post, especially since I've found what appears to be an easy and stable option.
Basically, by using the latest MPC-HC decoder, you can get DXVA support on XP and Vista. I have my H264 in MKV containers, so it's all set up through DS playback.
All you need is the right hardware: nVidia 8 series or ATI 2x00 series or 3x00 series
The decoder can be found here:
MPC-HC decoder. Just register the decoder:
regsvr32 c:\%pathtodecoder%\mpcvideodec.axThen, configure DS playback in MC12.
I set up my MKV source to use the Haaili Media Splitter and the MPC - Video Decoder. Make sure you check the option for "Enable DXVA" on the MPC decoder configuration dialog. Then choose your renderer (I use VMR9).
That's it. If you H264 sources conform to the general specs, you should get DXVA on XP and Vista. I've only test XP, under Service Pack 3, so YMMV. Kind of cool to see MC12 playing back a video @ 1920x1080 with the CPU at 0%!
More info, in general, about the decoder and limitations for the respective OS and Renderers can be found here:
MPC - HC About