During play press the 'SPACEBAR' to view the options screen. Move down to the VIEW GAME SCREEN option and push the JOYSTICK LEFT and hit FIRE. The game screen should appear. Center the joystick and press fire again. The option screen should now reappear. Move down to the SOUND ON/OFF option and push the JOYSTICK RIGHT and hit FIRE. Now go back up to the VIEW GAME SCREEN option and push the JOYSTICK RIGHT, while holding the joystick in this direction hold down the RIGHT MOUSE BUTTON and press the LEFT MOUSE BUTTON. The option screen should change to cheat mode. Now go to resume game and press the fire button. You now have unlimitted shields and can select any level on the options menu. If it doesn't work repeat the whole thing over (start by resuming the game and pressing the 'SPACEBAR' to go back to the options screen.
NOTE: This next cheat only works with the full price game not budget release. On the option screen, type "AL YANKOVIC" then press 'F5' for an effect. Also pressing 'X', 'C', 'T' and 'SPACE' keys produces a cheat screen.