For the demo
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During the game
NUM LOCK     = Invulnerability

Command line parameters for TEKD1.EXE
(ie TEKD1 NOENEMIES to start the game)
NOENEMIES    = Removes everyone but you
NOGUARD      = Removes the guards from the game.
NOCHASE      = Removes the shooting enemies from the game.
NOSTROLL     = Removes the innocent bystanders from the game.

For the full game
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ALT+SHIFT+G  = Invulnerability
ALT+SHIFT+W  = All weapons, lots of ammo, and all keycards.
ALT+SHIFT+J  = All matrix symbols that you would normally collect by
               killing the bosses.