Add '-cheatenable' into command line of the game (mgsi.exe -cheatenable) and use the following keys in game:
Code - Result
F2 - God Mode
F4 - Unlimited Ammo
F6 - Observe Mode
F5 - Normal Mode
F7 - Fast Level Restart
First Person Mode
For those of you who dont have the Info Booklet, like me, probably dont know that YES YOU CAN walk and shoot in first person view... just tap X (or whatever your first person view button is...) twice... in short: