And for a free cross platform media player that doesn't require any separate codecs to be messed with, try VLC..
http://www.videolan.org/vlc/
I've found it to be great, and can be a lifesaver. For example, a buddy was headed overseas and had a bunch of video files, but couldn't play them or get the codecs to work on his laptop. In 5 mins I had him download VLC and he was watching the videos with no problem, just in time for his flight. Simple and easy.