If you need to change the default in Vista Media Center (prior to TV Pack) and found that the XP Media Center Decoder tool no longer works for you then I have created a small utility called VMCD that helps with this... more details at
http://mediacenterexpert.blogspot.com/2006/07/vista-media-center-decoder-utility.html
Edit: New updated utility available that works in Vista, TV Pack and Windows 7
http://thedigitallifestyle.com/cs/blogs/garry/archive/2009/04/14/announcing-mcdu-the-media-center-decoder-utility-replaces-vmcd.aspx