grab GSpot -->
http://gspot.headbands.com/
the 2.51 beta version, not 2.21 !
use GSpot to open file that can't be played properly.
see what it says. (paste it)
in "proposed codec solution and tests --> MS A/V" press 1, and then 2. See if it's rendered properly.
then in "proposed codec solution and tests --> GSpot --> Vid" press 1, then 2, and for every render option that gave you the result "Video Render OK" right click and "play this graph". See if it renders ok.
If in any case the render is succesful, see what codec/filter was used, and paste it here.
Also in GSpot you can go to "System --> List Codecs and Other Filters" and check if there is anything marked red (red = problem).