Try deleting rgl_config.txt in \Documents\Mount&Blade Warband\
If you still have the problem, edit rgl_config.txt (it will be recreated when you enter the game) and change
use_secure_connection = 1
use_secure_connection = 0
Also, verify that the cd key in windows registry matches the cd key from the taleworlds website.
From the command prompt, type
Navigate to Hkey Current User>Software>MountAndBladeWarbandKeys