Like Halo CE PC, this game is considered abandonware and as such you can't find it on steam.
The best place to download the game is from Project Cartographer
Click on the top download button to get the game (Full Game Installer + Mod)
WARNING, I've only tested the campaign in offline mode. They seem very strict about cheats, so I can't guarantee the game won't ban your account for even cheating in the campaign in online mode. Cheat online at your own risk.Here's the table.
-Shield/Health Lock
-One Hit Kill
-Stealth
-Ammo Lock
-Grenade Lock
-No Shield Recharge Delay
-No Damage Indicator
-Values
-EXTREME GAMEPLAY (LEGENDARY++)
(UPDATE) The stealth should disable when the cheat is disabled, so you don't have to do that bit of funkiness from the original cheat. Also Grenade Lock will reset grenade values to 4 if the cheat is activated while you have no grenades.
(UPDATE 2) Health Lock will now be disabled while EXTREME GAMEPLAY is active.