I have downloaded both parts of the NWN game, but when I click on the .exe file it brings up the start of the installation process and tells me "Installer files are missing - Please make sure you have downloaded all necessary parts"

I have attempted re-downloading the game and still get the same message.

Any idea what's going on?

Just to be sure, are you talking about the first NWN? NWN1 has 2 parts but NWN2 has 3 parts.

Are you using Galaxy? Installer? May sound stupid question because if you downloaded the parts you are not using Galaxy but still i have to ask.

A few other people was asking the same thing so I am starting to think it is not a user problem. Sad part is I dont remember any of them explaining how they resolved their issue.

Only solution i can think of is to run the installer as admin and then choose a path other than program files. Like C:\Games\GOG......

Goodluck. And please write if you solve your problem.

Try using GOG Galaxy (https://www.gog.com/galaxy), it's the optional client for GOG. Once you have got that installed, you can install NWN from there, and it shouldn't have any missing installer problems as the files are all downloaded and installed for you.