pym66: I have Windows 7 64 bits.
I checked the topic about videos issue, I shut down the Anti-aliasing of my Graphic Card. I tried to change the scalevideo from 1 to 0 in the gothic.ini file but I can't save the change ...
Thanks

Paradoks: I have little experience with Win 7, but I think that you will have to run notepad as administrator to save the changes in that file.

Paradoks: You should run the notepad as an administrator, and then locate the file and open it.

pym66: Thanks to you, I success to change the INI file. But it don't make the trick ... The intro video works but not the Sleeper video ...
Another idea someone ?

Paradoks: I don't know a solution, but there are a few possible work-arounds for this problem. The first one is renaming the GreatPrayer.bik file in the _Work\Data\Video folder. Just change the name into something like GreatPrayer1.bik or something similar. I tried this, and to my surprise the game did not crash with "file not found error", but continued like nothing happened (except not playing the video off course). I have playerkit installed however - I don't know if the game crashes without it.
As a result - you should be able to proceed, but unfortunately you don't see the video - but you can open that file with RAD Video Tools or just see it on YouTube (I didn't search but I'm sure it's out there somewhere).
If that doesn't help -you can cheat your way out of this, and jump directly to Act 3 (it begins right after this cinematic). I will describe it in detail if previous way does not work.
Neither of these will allow you to see in-game cutscene however - perhaps someone can find a proper solution.
Just a few questions that may be helpful - can you see the credits (option in the main menu) after you start the game?
The size of GreatPrayer.bik file should be about 58 MB (that's the size in my Polish version) - if it is significantly smaller it may be a corrupt file - in that case reinstalling should help.
I hope this helps.

pym66: Thanks to you, I success to change the INI file. But it don't make the trick ... The intro video works but not the Sleeper video ...
Another idea someone ?

Arkose: UAC redirects protected file writes to another location. You should be editing the ini located within C:\Users\<user>\Appdata\Local\VirtualStore (where <user> is the username you are logged into when playing this game). The folder structure reflects where you installed Gothic to (by default it will be in VirtualStore\Program Files (x86)\GOG.com\Gothic on a 64-bit OS, or VirtualStore\Program Files\GOG.com\Gothic on a 32-bit OS). Note that AppData is a hidden folder (you can enter it in the address bar to get there manually). Once you edit the VirtualStore version your desired change will take effect.
The VirtualStore method applies to most other older games too unless you are running them with administrator elevation (which very few need) or have installed them outside Program Files.