type "oh dear" (small letters) in the main menu.
 
 After that you will have various cheats.
 Single player only.
 
 alt1=full ammo
 alt2=upgrade current weapon
 alt3=full energy
 alt4=indestructible
 alt5=wow
 alt6=mission compleate
 alt7=deactivate enemies
 alt8=heli mode
 alt9=tank mode
 alt0=speeeed!