Get this and install it, most all WMP playback problems will be gone in Vista.

 

http://www.jtow.net/users/triess/

 

Remove Quicktime and any other add on codecs you may have installed in add/remove programs before installing, this package has a working quicktime codec.

Thanks for your response.  I had already installed the Vista Codec Package.  Someone had recommended it to me.  But it didn't fix the problem, however.  I only installed QuickTime after the fact, because I wanted to see if it worked.  When Quicktime did play the mpeg1 files correctly, I knew the files aren't corrupt.  And since Windows Media Player plays all other files correctly, I know it's not the media player - especially since Real Player from Real Networks also can't play the mpeg1 files correctly.  But thanks for your suggestion.  Is there anyway to uninstall ALL the Codecs and then re-install the correct ones?  Or any other ideas from anyone??

You can use add/remove programs to uninstall any pachages you installed, there was a program called codec sniper to clean up any left after add/remove, that I used on XP, not sure if it works on Vista or if there is a version that does.