What's wrong with my MPEG 4?

For some reason my computer will play any videos except those done in MPEG 4. Gspot says I have compatible codecs but they still won’t play. I’m getting an error code of 0x80040200, which is Microsoft speak for “You don’t have the codec.” The problem could be too many MPEG4 codecs but I have no idea how to remove them or which ones to remove. Any help would be greatly appreciated.