mtraun: I've still got this problem and it's really annoying. I've reinstalled and I get the same problem. How the hell do I fix this!!!! Come on GOG!
From thwitcher.com support page:
Error with invalid or incomplete game version occurs in cases when wrong patch version is being applied to the game, or when the game for some reason cannot find all the necessary files. It can happen if the game was installed in catalogue named using diactric signs, which are not recognized with currenty Windows language settings. It is also caused by changing the default location of My Documents folder, installing game on Windows Vista or Windows 7 without administrator priviliges or just simply lack of space on C: HDD. In order to solve this problem, one has to:
1) Download and install the patch according to guides that can be found on:
http://en.thewitcher.com/patch/
2) Before installing the patch delete all dzip files beside pack0.dzip and troll.dzip from CookedPC folder in The Witcher 2 installation directory.
3) If the above does not help, please uninstall the game.
4) Make sure that all files from the game's folder were deleted.
5) Make sure that My Documents folder is in Windows' default location.
6) Run the installer as the Administrator
7) Install the game in folder that doesn't include diactric signs.
This covers anything I can think of that would be causing the problem. It doesn't mention to make try installing with launcher closed. Oughta try that too.