There are so many media players you could choose as your default. Windows Media Player, VLC, QuickTime, etc. Chances are that you have a favorite. You can set the default with these steps:
1. Open the Control Panel.
2. Open Add or Remove Programs.
3. Click Set Program Access and Defaults on the left side.
4. Scroll down to where it says Choose a default media player.
5. Select the preferred player. Click OK when done.