To enable Call of Duty cheats in the demo, modify your CoD Single Player shortcut and add the following text in the target field AFTER all the other stuff:

"+set monkeytoy 0 +set sv_cheats 1" (without the quotes)

Then start the game using that shortcut and press [~] to open the console, in there type:

god - God mode
give - Gives indicated item
give health - Full health
notarget - The enemies won't shoot you
noclip - You can go straigh through walls
give all - Gives all items (DO NOT TRY THIS, crashes the game)
give ammo - Gives ammunition (DO NOT TRY THIS, crashes the game)

NOTE: If after you type one of these codes in the console it tells you that "sv_cheats are read only", then try typing this into the console: "setrandom sv_cheats 1" (without the quotes). If you, on the other hand, are getting "cheats are not enabled on this server" message then type: "spdevmap burnville" (without the quotes).