Launch Winamp.
Go to "File" then "Play File."
Select the MP4 file you want to play and click "Open" to begin playback.
Find any MP4 file in Windows and right-click on it. Go to "Open With" then "Choose Default Program" (In Windows XP select "Choose Program").
Select Winamp from the list of recommended programs. If Winamp is not there, go to "Browse" and then navigate to "C:\program files\winamp" and double-click on "winamp.exe."
Click "OK." Now whenever you double-click on an MP4 file, Winamp will be used to open it.