If you have Windows Media Player 11, the problem is -- it sucks.
Just kidding ... sort of.
WMP used to be great. I had no problems with it set as my default player. Most everything except Flash or Quicktime video, obviously, played in it without trouble, and it would run most video on news sites, etc., without actually having to open in the player itself.
Anyway, point is, since using 11, however long ago that started, a lot of stuff won't play as is. I get the green screen with audio or nothing at all sometimes.
I've scanned around online for solutions and tried most of them with no success. Tried a wide variety of codec packages with no luck, since that seemed to be the most common answer provided.
It also seems to be next to impossible to uninstall WMP 11 and go back to an older version that would be less problematic.
About the best thing I've found to do is reduce the video acceleration. This is under 'Now Playing', 'More Options' and 'Performance,' in that order. I don't know if this will help you with your problem or not, but the downside is you can't maxmize the screen.
Anyway, to sum up -- WMP 11 sucks, but I don't feel like having to use something else.