Windows Media Problem

I got a mp4 file and my windows media 9 wouldnt play it, nor winamp or quicktime, so I upgraded to windows media 11. It wouldn't play it either but it worked with all of my older videos. Then I downloaded 3ivx and the mp4 file worked. When I tried to open my old videos in windows media 11 it says "The application or DDL C:/windows/system32/wmpdxm.dll is not a valid windows image. Please check this against your installation diskette". Then when I tried to open the mp4 video again it gives me the same message. I tried uninstalling windows media 11 and that didn't work. Whats wrong?