I Fixed this problem on my computer (I think). The way I did it was that the reason that the play/pause button opens WMP is that It is tied to a specific type of file. So when the button is pressed it launches that type of file and so opens WMP. To fix this you need to change what program that file type is tied to. To do this you need to go into control panel ~> Programs ~> Make a file type always open in a specific program. Then and find the correct file type, (I am not sure which type it is) switch it to your primary media player from WMP and it should work. I am not sure if this is exactly how it works, But I have a wireless keyboard with a media button and this was the way I got it to not open WMP.
