Click Start - Control Panel - Default Programs and select Set program access and computer defaults.
Click on Custom, if nothing shows, change it to something else, then click it.
Under Choose a default media player, deselect Enable access from this program from Windows Media Player and Windows Media Center and any others (like iTunes)
You could also try removing the following Registry Keys:
REMOVE PLAY WITH WINDOWS MEDIA PLAYER FROM CONTEXT MENU.
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Audio\shell\Enqueue]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Audio\shell\Play]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Image\shell\Enqueue]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Image\shell\Play]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Video\shell\Enqueue]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\Directory.Video\shell\Play]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\audio\shell\Enqueue]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\audio\shell\Play]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\video\shell\Enqueue]
[-HKEY_CLASSES_ROOT\SystemFileAssociations\video\shell\Play]