When the title screen pops up, type in the followings:
|Start At Level 1||level one please|
|Start At Level 2||level two please|
|Start At Level 3||level three please|
|Start At Level 4||the last level please|
|Energy refill||give me more health please|
|Extra time||give me loads of time please|
|Invincibility||i do not want to die please|
Also, type Neil Harding in the high-score list and something funny happens...
And finally: start a 2-player game and hold down both fire buttons, then hit Esc like crazy until the credits go bonkers and you get infinite lives.
The last two need someone to test them and write up what happens.