On the high score table enter any of the following for your initials:
"KDJ"------For infinite lives
"PJA"----Infinite smart bombs
"RLH"------------For acid men
"JGL"------For full firepower
"RAB"-----------Invincibility
"HSC"--Turns the screen black
Note that you enter the first two letters as normal, then press and hold '5' on the numerical keypad, then enter the third letter to register the cheat.