It's possible another software install (such as video editing
software) may have changed the DVD decoder "merit" to one which is not
WMP/Media Center Compatible.
You can check to see which decoders are compatible using DecCheck :
http://www.microsoft.com/downloads/details.aspx?FamilyId=DE1491AC-0AB6-4990-943D-627E6ADE9FCB
On XP, you can change the decoder merit to one which is listed as
working. On Vista you can't (I'm not sure of a good other-way to fix
that)
HTH
Cheers - Neil
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Digital Media MVP : 2004-2008
http://mvp.support.microsoft.com/mvpfaqs