Here is the easy way. You need a GFX card capable of it, eg. nVidia Purevideo.
Ensure you have your cl264dec.ax registered.
(if you don't have CL264dec.ax, just download it for free Cyberlink PowerDVD Trial Version has it)
In DVBViewer when playing DigitalTV, goto Options and ensure Vista Enhanced Video Renderer is selected as Video Renderer (under Hardware). Also select the H264 PDVD codec as the H264 decoder.
When video is playing. -> Options at the top -> Filters -> PDVD H264 Filter -> Tick the DxVA box and rebuild the graph (or restart DVB viewer).
Do the same, except make sure you have .Net 3.0 framework installed and the EVR.dll and DxVA2.dll registered.