Maybe this could help: http://www.taleworlds.com/main.aspx?dir=convertserial.aspx
It won't, not at this stage.
If you purchased the game retail (in a box) you receive a Steam activation
a CD Key or Serial Key. If you don't have a Steam account, you'll need to set one up and then use that key to activate the product in Steam.
After you have activated your key in Steam, you have 2 options:
1) Allow Steam to download and install the game. You'll then play through the Steam client.
2) In your Steam library, select Warband and then click CD Key to get an actual game serial. If this key is also 5x5, you can then use the convert serial link above to turn it to 4x4 and use that key to activate the Taleworlds.com downloaded version of the game. After doing so, register your key to your Taleworlds profile here (for safekeeping) and feel free to forget about Steam.