Because the game was released in 2007 and master servers have shifted, there is no active central enforcement team banning players for level exploiting in the original CoD4.

For Xbox 360 and PS3 players, there are "modded save files" available for download. You download a save file to a USB drive, use a program like Horizon (Xbox) or BruteForce (PS3), and resign the save to your profile.

Click the "Easy Upgrade" button. You should see a success message: "Easy Upgrade completed successfully!".