After a lot of messing around with codecs, I've successfully gotten VC-1 videos to playback using DXVA hardware acceleration in Windows Media Centre on Windows 7. CPU usage is 8%-15% on a 2.13Ghz Core 2 Duo. This is down from around 80%-90%. GPU usage is around 10% on an NVidia GT440.
Don't bother with the Microsoft DMO codecs for VC-1 as they don't use DXVA (at least they don't with my NVidia GT440) and use more CPU time than software based ffdshow.
The ffdshow DXVA enabled codec for VC-1 seems to crash on the VC-1 streams I tried.
Download Media Player Classic MPHC Stand Alone filters from here
. Make sure you you download the 64bit version if you're using 64bit Windows Media Centre.
Extract MPCVideoDec.ax from the zip file to a safe location and then register it from the command prompt using "regsvr32 MPCVideoDec.ax".
and select "MPCVideoDec" under xx-bit decoders/VC-1.
Everything should work straight after that (no reboot required). If you encounter any problems then you can use the DSFilterTweaker to switch back to the Microsoft decoder and "regsvr32 /u MPCVideoDec.ax" to deregister the MPC codec.