Press F2+F3 at the Main Menu. CHEATS ON will flash on the screen.
Press + and BACKSPACE for the cheat prompt, then type in these codes:

TURBO - Turbo mode
EVERY LEVEL SAVE - Enable save after each level
ALLOW SINGLE - Single player on multi-level
PAPARAZZI - Screenshot - press insert, will save in C:\TEMP\PIX
THE WHOLE OF CREATION - All cheats off
SCOUSER - Enable playtester keys

Playtester keys:
Shift-F9 - Skip level
F1 - Ammo cheat
F12 - Health cheat