The media player which is most frequently recommended on AB is the same one which has been recommended on the BBC World Service technology programme, BBC TV, Radio 4, several computer magazines and countless websites. It's called VLC and you can find it here: http://www.videolan.org/
VLC is far better than Windows Media Player. For example, it doesn't constantly report that it hasn't got the right codec to play a file. You can use it to play all your media files, including mp4.