As far as i am aware this is not possible and if there was a way to change it then it would probably do more harm than good...
Although this isn't quite what you were after, the space bar does still work providing you have the player list in focus.
e.g.
play a list of tracks and make sure you are showing the track list in windows media player (right click the windows media screen and select show list) once you click in that list somewhere the space bar should work.
If you are on a desktop and would really like the shortcut, i would suggest a new keyboard with the media buttons.
Regards,
JDobbsy1987