Metroid Zero Mission takes place before the NES Metroid.
http://forums.g4tv.com/messageview.cfm?catid=8&threadid=515808&FTVAR_MSGDBTABLE=&STARTPAGE=1
This link will take you a presenation of facts that prove the game is a prequel beyond a shadow of a doubt. If you have any questions, or wish to change the IMDB description for this game, please discuss it here first so that you can see why the game is a prequel to Metroid.