In that particular codec without DRM and without other videos or graphics intensive action going on at the same time.
You want a panel of videos (it happens more than you think) or to apply a shader to the video (e.g. during adaptive streaming) or use a new codec or use DRM or run it at the same time as Windows Aero Glass, then it'll all be happening in software.
Which is why Microsoft puts their efforts into making software playback good and not concentrating on hardware playback. Because hardware video decompression sucks.