When the latest media player won't play files, I use the legacy player located in the same folder as the latest. Just right click the windows media player icon, select properties, the select find target. Once in the media player folder you will see the icon to the legacy 6.4 media player. You can right click it and send a copy of it to the desktop for easy viewing later. The try to open your media with it.
If that doesn't work, try using the divxplayer, or another freebie.
I recommend the following site for freebie players to help play your media,
http://www.zdnet.com
hope that helps,
BB