Try backing up the M&B folder where you found the rgl_config, then delete that folder. If the game starts correctly, the folder will be recreated.
You needn't deal with this whole folder (the other files are just screenshots, edited scenes, exported characters and the like). Just delete rgl_config.txt (found in C:\Users\<UserName>\Documents\Mount&Blade Warband\ )
If that is insufficient, the folder to delete is the hidden folder at C:\Users\<UserName>\AppData\Roaming\Mount&Blade Warband
Is there a way to retrieve my CD-Key or reinstall my game without screwing anything up? THanks.
If you reinstall, you needn't worry about your CD/serial key. It will still be stored in Windows registry and will be applied to your reinstallation (but reinstalling will also NOT touch the configuration files mentioned above, which are most likely the actual problem...so deal with the steps above before reinstalling).
If you want to find your serial key:
Hit the Windows Key and R, which will bring up a Run... window.
Then type "regedit" in that window and press enter.
Once there, navigate the tree to the location I indicated:
HKEY_CurrentUser->Software->MountAndBladeWarbandKeysOn the right hand side you should see a list of things like serial_key and activation code.
(I'd then recommend recording it here on the website
for easy future retrieval...actually, which it appears you have done, given your Game Ownership Badge)