Enable cheats:
Start the game with the -drake -strega -amitab -fuzzyhead -mice command line parameters. An easy way to do this is to right-click on the desktop shortcut, select 'Properties' and add these to the 'Target' field.
Now press the following during play:
F7 God Mode
F8 Invisibility
F10 Automatic mission completion
F11 Manual mission completion
F12 Fail mission
