At the password screen enter the code: 6716127
After doing that it will take you back to the main menu. When you try to enter another password it will prevent you from going into the password screen. Instead type a number between 1 and 21 and press enter. This will let you select a level you wish to start on.
There is one more cheat:
Hit "P" during gameplay to pause the game. Now hold down the keys "L A C" on the keyboard all at once. This will enable/disable god mode. You are now invinsible and can advance quickly through the levels by holding the ALT key.