Press T to enter talk mode during game play.
Once there just type any of the following:

ammo = Gives full ammo
god = Gives god mode
guns = Gives all guns

Access All Chapters
To open all chapters of the game, run REGEDIT and find the key:
[HKEY_LOCAL_MACHINESOFTWARE\Monolith Productions\Contract Jack\1.0]

Change the value of "EndGame" to 1.
Note: Always make a backup before editing the registry.