I may also have a possible fix to this:
If you play Warband through Steam, and you get a popup with your Warband and Napoleonic keys and never did anything with them as of yet, try the following:
1) Download mb_warband_network_test if it isn't already in your Warband folder (steam/common/mbwarband, or something like that). I attached the file to this post and also uploaded it here:
http://www.fileswap.com/dl/Yq9PWiyYV/2) Extract the file to your warband folder (if needed) and run the exe file and it should give you an activation popup.
3) Choose activate and type in the Warband serial (not the napoleonic one)
4) It should give you a confirmation that the serial was accepted - now open up Napoleonic Wars
5) Connect to a server - it will give you a popup asking for your NW serial key
6) Type in your NW serial key into that box (no dashes or spaces) and it should allow you to connect.
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