On the main menu, hit Up, Up, Down, Down, Circle, Down, Up, Circle to activate the cheats menu. Input the following codes, which can be turned on/off after they are entered:

DANCINGFEET - Disco Mode
BURNERSREVENGE - Terror Mode
BURNINGBULLET - Bullets Set Fire
KEEPYOURHEAD - No Heads
ISEEYOU - Better Visibility in Smoke
WHATWASTHAT - Hi-pitched Voices
FRYINGTIME - Long Range Stungun
MINIFUN - Minigun (Replaces Pistol)
ULTIMATEPOWER - MK3 Assault Rifle With Unlimited Grenades
ZEROTOLERANCE - MK4 Pistol
KEYTOTHECITY - Unlocks All Levels And Emergencies