This is something I struggled with for quite some time so I figured that I might as well make a guide on how to do it.
First off regardless of your operating system in the 64 and 32 bit sense, Uninstall ALL of your codecs.
Then open up notepad.
Paste this in
Windows Registry Editor Version 5.00
Save as, mkv.reg
any file type.
Doubbleclick on the file and add it to your registry.
Restart your computer.
For windows 64 bit go here and download and install this codec pack.
For windows 32 bit go here and download and install this codec pack (the full version).
Worked for me on windows Vista Ultimate 64 bit, Windows Vista HP 32 bit, and Windows XP MCE 2005.
Hope this helps some people out who don't want to use a 3rd party player when you can just do it in media center
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