I had this same issue with Baldurs Gate 2. Now I'm having this issue with Neverwinter nights
I would try to download the game and it would tell me the set up files are corrupted.
I can't download the GOG downloader since it crashes whenever I try to launch it. Every other game I have downloaded just fine but this one isn't.

I finally figured out a way to download Baldurs Gate, I had to rename the .exe files.

What can I do for Neverwinter nights?
Any ideas?
What do I rename the files if I needed to rename them?

Thanks for the help!

http://www.gog.com/en/forum/neverwinter_nights_diamond/help_with_neverwinter_nights_installation
This might help you. I noticed this after I was contemplating on whether not I should post that I have the same problem. I'm going to try it, and get back to you.

It doesn't really solve the problem, so I too am stuck with this problem. Any fixes?

Maybe this will help. My nephew had a similar problem and I found a solution and it appears it can be used with any GOG game. Of course, this is assuming the files are not corrupt.

Any time the "exe" installation file includes one or more "bin" files, they will have the same filename. For example, Painkiller Black also uses one exe file and two bin files with the following names:

setup_painkiller_black.exe
setup_painkiller_black-1.bin
setup_painkiller_black-2.bin

Notice how they have the same filename. The only difference are the -1 and -2 at the end of the bin files.

You can change the names of these files to anything you want as long as you don't change the -1 at the end of the first bin filename, -2 at the end of the second bin filename... and so on since some games include more than two bin files.

So, if I wanted to use the filename "Painkiller_Black_Setup" I could rename the files as follows:

Painkiller_Black_Setup.exe
Painkiller_Black_Setup-1.bin
Painkiller_Black_Setup-2.bin

The same principle applies to Neverwinter Nights. If the names of your installation files don't match, you will be told one or more of the files is corrupt. So check the filenames and make sure they match using the above as an example. If you don't know what the original filenames were, then just name them as follows:

NWN_Setup.exe
NWN_Setup-1.bin
NWN_Setup-2.bin

The exe file will be the smallest of the three files.
The -1 bin file will be the largest
The -2 bin file will be the remaining file