On the title screen, type "R U CALLING ME A PINT A POFF?". You should be able to kill any opponent with the 'DEL' key, or try typing any of the following on the title screen:
"LEVEL ONE PLEASE"-----------Go to level one
"LEVEL TWO PLEASE"-----------Go to level two
"LEVEL THREE PLEASE"-------Go to level three
"LEVEL FOUR PLEASE"---------Go to level four
"LAST LEVEL PLEASE"---------Go to last level
"GIVE ME MORE HEALTH PLEASE"--Lots of energy
"I DO NOT WANT TO DIE PLEASE"---Invicibility
Play two player game and get both players killed until you have one credit left and the "continue" message is displayed on both sides of the screen. Press both fire buttons simultaneously and you will get infinite lives.
On the high score table, type "NEIL HARDING" as your name for something funny.