For all those out there that need a bit of beginners help getting their content to play, I just wrote a quick Guide to Codecs in Vista Media Center.  It covers basic information and troubleshooting for getting third party content to play.  Includes AVI, MKV, and more.

I hope this helps some of you get started.  Understanding all the crap that happens to your system isn't easy.  I wish Microsoft would include better tools for managing DirectShow codecs.  I also wish OEMs and software manufacturers would do a better job writing their codecs if they are going to ship their products setting a high merit on their filters.  Various companies do this, and their codecs don't play well with other applications.  It does nothing more than cause problems.  The second biggest problem is small companies doing video editing and conversation programs.  They include so many small filters and just go about registering them without a care in the world that it might screw up other applications.

Anyway, I hope that information helps some of y'all get started.