Tip Of The Day: FFDShow
FFDshow got its claim to fame, at least in my little world, for being a DirectShow filter that would allow you to scale DVD content and apply various filters to it. Of course its other great feature is that it will play XviD and Divx with nary an issue.
The Sourceforge page linked below has links to the forums and the latest files for download. FFDshow is a lot better then the troublesome codec packs that leave all sorts of crap hanging around
I have yet to test FFDshow with H.264 content or .mkv containers. What are you using to play back your H.264 encoded content?