how do i change the fn keyboard function on a asus k50ab not

when hold fn and press the down button(fn + down = play/pause) opens windows media center where can i change it to winamp for exemple?

You can't. The only thing you can try to do is to change your default media player to Winamp. Look under Control Panel -< Add or Remove Programs -< Set Program Access Defaults. If you can change the Media Player to Winamp, you're all set. If Winamp doesn't appear there, you might want to try this:
http://forums.winamp.com/showthread.php?postid=2042822
With this you basically create an uninstall information for winamp, which is needed for it to appear under SPAD. After that reboot and it should be there, and you should be able to set it as default, and even restrict access to media player (which you should do anyways XD).