Microsoft invalidated a few CODECS so you need to find the decoder (CODEC) and install it again.
Gotten worse about not playing some mpg's that played fine a year ago? Now WMP10 insists on connecting to internet every time, and has begun to say "Codec download error" or other similar message, or just "File corrupted". Yet with other players (VLC, MPClassic, several dvd players it works fine and no amount of checking the codecs, repairing, etc will make it play the video again. I hear silly stories of WMP11 being worse.. Thanks for any tips, etc. Tried restoring to several weeks ago, no help. Maybe I'll just change all the default open with stuff away from WMP, hide the icons, say 'Adios, error message!'