One of my computers actually got this issue in I.E., some video files just showed up as a red x all of a sudden. I just used Firefox on that computer and then re-built it with Windows 7. Could not find a fix for it, every video plug-in was installed.
You can try to install the following and see if this helps you: MS Silverlight, QuickTime, RealPlayer, Flash Player, Shockwave. Or do what I did, install Firefox and use that.