Sometimes the automatic codec download works better if you try to open it WMP classic. I have a hard time getting any codec to download with WMP 10, but if I open it in classic it downloads fine.
If that doesn't work, GSpot will work.
I used a program called "video inspector" a couple of months back and it worked great.
It told me exactly what audio/video codecs were being used.
KC Softwares - Video Inspector