Hit ALT and at the same time to be able to enter these codes:

HELLO = Shows message: Hello?
SLUGS = All Ammo
ARNOLD = All Guns
WHOAMI = Shows your name
GARBLE = Garble cheat codes toggle
VERSION = Shows version
ICANTSEE = Show viewscreen
SUPERUZI = Get the Super UZI
COUNTERS = Shows current co-ordinates
ILLBEBACK = Skip that Level
WILLNOTSTOP = Invincibility