Use Windows Explorer to navigate to the folder where you installed the game.  Locate the file hitmancontracts.ini and edit it with Notepad.  Add the two following lines to the file:

ENABLECONSOLE 1
ENABLECHEATS 1

Save the file and then launch the game.  During gameplay press [Shift] + [Esc].  This will display the cheats menu and you can select from such cheats as god mode and all weapons.