The game checks the Mount & Blade key rather than the Warband key, which if you haven't had 1.011 installed since you last reinstalled Windows will cause this problem because the key won't exist.
The quickest and easiest way to solve it is to install the
latest Mount & Blade and activate it with your serial. You should then be able to play Warband
The harder fix (or if the above fails), is to edit the registry and manually enter your serial. *NOTE : I strongly suggest backing up the registry via file > export before you start mucking about in it*
Start > Run (or search on Vista) > Regedit (type and hit enter)
First thing to do is scan the registry for the following key:
HKEY_CURRENT_USER -> Software -> MountAndBladeKeys
If it doesn't exist you'll have to create it manually. Navigate to HKEY_CURRENT_USER > Software and right click, then choose "New Key". Name it exactly (case sensitive) MountAndBladeKeys.
Select the MountAndBlade keys and over on the right you'll have either a blank pane (if newly created) or some values already. If serial_key already exists but is empty right click, edit and enter your serial code (complete with the dashes, eg XXX-XXX-XXX).
If the panel is blank, right click in it, choose new and then string value. Call it exactly serial_key. Then right click, edit and enter your serial key.
