Ebon-Hawk: I have tried to play the bonus videos available for downloads as extras for Witcher 2 gog.com version (particularly the launch trailers)
While I have no problem viewing the .mp4 files, I am wondering what codec is required to play the video of the .avi files.
I have tried Windows Media Player and WinAmp Pro on Windows 7 x64 SP1 and get sound only.
I'd just try VLC Media Player. That thing plays whatever you feed it.
I'm not sure about the used codec though - I don't have the GoG-version myself.