Various Cheats:
Edit the properties of the Metal Gear Solid File to start with -cheatenable. Example, make a desktop shortcut, edit the properties to look like MGSI.exe -cheatenable

Now press one of the following keys during play to cheat

F2 - God mode
F4 - Infinite ammo
F5 - Normal view
F6 - Observe view
F7 - Fast level restart
F8 - Texture mode 1
F9 - Texture mode 2
F11 - Texture mode 3
F12 - Texture mode 4

(Note: The God Mode and Infinite Ammo only work in certain areas, in other areas God Mode will only refill your life.)

Alternate Method:
Create a file in notepad called mgs.bat
all the file should contain is (assuming you used default install):

cd\progra~1\metalg~1
mgsi.exe -cheatenable

Save the file and run the game from that file.

when in game press:

F2 - god mode
F6 - observe mode
F5 - normal view mode
F7 - Quick level restart