can't play video

I am having a problem making a video, I have tried all the available formats.
I am using 2.56b in a PC XP
Every thing has bean working great and then all of a sudden when I render an animation
I get a different icon for the file and I can’t play it.
Windows media player gives me this error message, The specified module could not be found. I tried reinstalling blender and have checked my drivers. Still the problem.
I can’t think of anything I have done to cause this.
Thanks for any help.

It seems as though if I set the resolution under the rendering dimensions to something that is not too close to the preset size it doesn’t render properly. If I set the preset first to a size close to what I want then then make changes things work,
go figger