The video landscape on the PC is still far too big a mess. There are too many codecs, and sometimes too many software providers making decoders (we found a dozen H.264 decoders and at least as many DVD decoders in 10 minutes of Google searching). Some are accelerated, some are not.
You can get acceleration for DivX, but only with the right hardware and only if you use DivX's player. H.264 acceleration is limited to a single graphics vendor and a single decoder, and it's quite easy to install other software that takes precedence over the accelerated decoder, destroying your performance. It's practically impossible to be sure of what codec a video file uses based on its filename.
If you want your video to look its best and run as fast as it can, you have to enable all sorts of settings in the advanced properties of your player (or players, plural), and those settings can be different between ATI and Nvidia cards. In short, Microsoft needs to seriously clean up this mess. Video codecs need to hook into a common framework, one that the graphics cards manufacturers can target for acceleration without needing to work with every individual codec maker on the planet.
