you need to check you cd key though and make sure that is is the 5 x 5 key and not the 4 x 4 one Steam issued when they ran out. If it is the 4 x 4 you can contact Taleworlds and I believe they will give you a 5 x 5 to play outside of Steam
both work just fine if they are actual game keys. I have a 5x5 from steam and a 4x4 from amazon that both play the game perfectly.
if you want to switch cd keys, open regedit and go to:
HKEY_CURRENT_USER\Software\MountAndBladeWarbandKeys Then click Serial Key, and copy the other key in.
Keep your keys in a notepad file for easy copy and paste, and safe keeping.
if you DON'T want to switch keys, just have a different non-steam install and play your characters with your old steam key.
if you ever launch the steam version of the game, it over-writes the registry with the steam CD key.
*I play crpg on a separate non-steam, clean crpg only install. I get much better performance and loading time this way.