Are all the file name extensions the same? MPEGs can come with a wide range of extensions, such as:
- .mpeg
- .mpe
- .mpg
- .m1v
- and so on
It is possible that not all of these extensions have been associated with Windows Media Player.
In Media Player, go to Now Playing, More Options, then select the File Type tab. Look for
Is the tick box grey or white? If it is grey, some file extensions are associated with other applications (such as QuickTime). Un-click it, then click it again (it will be white this time) and click on "Apply".
Does that fix it?