Hidden Options
     The  password  for the hidden option VS. Computer in the game is:
DOAKSIM

Other Passwords
     Type the following codes in the "Password Entry" section:

KIKMAHP
DKRORCR
STKZJGH 

They activate: 

1) Free Play (Infinte Continue)
2) Allow access to all boss in Boss Mode
3) Allow access to all levels in Drill Mode
4) Allow access to all CPU characters (Citizens, Harry, etc...) in 
   VS CPU mode