I'm having a slightly different problem as I do have a NW serial key but after inputting it I got a message saying that it was incorrect.
I know the key works because I already used it on another machine so I probably just got part of it wrong but the problem is now I can't input it again. I'm running on a Mac so I can't change it in the registry as I believe one can in Windows.
So, my question to those of you on this thread is; How did you go about changing the serial number in NW after having put in the wrong (MB) serial initially... assuming that's what you did.
1. Click Start > Run
2. Enter regedit
3. A new window will open, browse to HKEY_CURRENT_USER > Software > MountAndBladeWarbandKeys
4. Search the entry, serial_key (serial_key_nw for Napoleonic Wars), make sure that the value is the same as your CD key. Also make sure that the key is separated correctly. Double click the entry and modify it until it is correct. Watch out for invisible spaces in the CD key.
5. If you are done with that, close the Registry Window and start the game