My friend ran to this problem after trying a few video editors (AviSynth, StaxRip, VirtualDub, DVX, SUPER, and AviDemux). They were supposed to be okay, all of them recommended by the afterdawn forum or free-codecs.com. However, one of them messed something up.
This i what I tried to do:
I uninstalled all video-related programs, even the ones that worked perfectly before this mishap. I've tried system restore, uninstalling all codecs, temporarily removing all Windows/System32 entries displayed by Gspot, removing (with drivercleaner) and re-installing videocard drivers, removing all video related program entries with RegSupreme Pro, trying different codec packs, even using VLC player that is supposed to use it's own drivers instead of the ones installed in windows.
All players (MPC, WMP, AviPreview, VLC) do resize themselves to the correct size, but the image is just plain black. And sound is playing fine.