When you reinstalled windows did you reinstall whatever programs you used to play media files.
Reason I ask is without anything else being installed on the PC windows would probably try to play any media file with media player even if the file wasn't supported. And if the file wasn't supported it might try to alter them to a file it could play. Perhaps before you were using different programs to play the files and just need to reinstall the programs?
Or could it be a codec problem as by default media player will not play films.