Currently I need to execute 'dxdiag.exe' and run the 'Display 1 | Test Direct3D' tests - at least partially before hardware accelleration will work within my apps (DVB Viewer and GBPVR). Until a Direct3D test is executed, the apps use a large amount of CPU and no hardware acceleration ... which is instantly fixed by running a test via dxdiag.exe ...
Of course this needs to be down after ever reboot - I have yet to investigate if there is an easier way to 'initialize' the hardware accelleration functionality. Maybe this is due to the DirectX 10 XP hack that I installed.
My other issue is, as suspected, the accelleration only appears to work on the primary display, and as i currently use my PC as a home PC as well as media center, and the ATI drivers do not allow configuration of what goes to which display, it could be quite awkward to get a nice working (wife friendly) solution.
At this stage I plan to go ahead with a fresh install of XP, to keep it separate from the current working setup, and experiment with getting the decoding working under that, and sorting out the best solution for a dual use PC.