Or:
http://www.gamingforums.com/showthread.php?t=96810
Warning, this involves modifying a game file, so make a backup.
These codes DO NOT work in muliplayer, so don't bother trying. The server has to have the codes enabled before you can even think of cheating (using this method) in a multiplayer game.
Step 1: Open your favorite hex editor (I prefer
http://www.chmaas.handshake.de/delp...xvi32/xvi32.htm )
Step 2: Open the CoDSP.exe file, found in the Call of Duty directory.
Step 3: Search for the TEXT string "sv_cheats" (without quotes).
Step 4: After the last "s" in "sv_cheats" you should see three square symbols. Click on the first one, then click on edit>overide string. Make sure HEX is selected, then type "25" (without quotes) into the box. Now save the file, and follow the demo cheat instructions.
And there you have it. Credit goes to a guy on the callofduty.org boards. He posted the instructions, apparently, and deleted it before I could see it. Luckily someone mentioned Hex Editor later on in the thread, and I figured it out from there.
Happy gaming!