I downloaded the game at a friend's computer in the army so I can play when out of home, but it keeps telling me my CD is in use and to try again in 5 minutes. Solution?

You need to send off for a CD key through the support section of GOG. If you are using someone else's copy, and therefore CD key, you're out of luck.

I was sometimes getting that message and sometimes not. I've sent off for my cd key from GOG, but I also found a cd key in my NWN folder. I'm not sure if that one is the same for everyone, which could explain why sometimes it's in use and sometime's it isn't.

Follow-up question: Where do I put in the CD key?

You replace the generic one you found in your folder.

I made the mistake of not actually replacing it or thinking I deleted it and putting it in the folder and having 2. Don't do what I did and make sure you check the NWN cdkey in the ini file you get and then checking it again in the nwn folder to make sure it matches :)