NEED HELP DOWNLOADING CODEC!!!?

OK, so I can't see videos on my Windows Media Player and I can't import them to Windows Movie Maker because it says I didn't download the XVID MPEG-4 (xvid) codec when I actually did. I can, however, hear the sound on WMP. If you could try to help me figure out this problem that would be great. It is urgent because I have a project due Friday and I need to be able to view the clips to edit them. Thanks so much for any help!