Quick Save registry edit (how to)
Run "regedit" (Go to Start > Run... and type in "regedit") and open the following folder:

HKEY_LOCAL_MACHINE/Software/Computer Artworks/TheThing/1.0

Right click on it, select New > String Value and use one of the following names for it:

PlayerInvulnerable - Unlimited Health For You NPC
Invulnerable - Unlimited Health For Allied Friends
FullWeaponEquip - Get All Weapons
DoInGameLoadSave - Load Game and Save Game options appear in the options menu

After you've created it, right click on it, select Modify and enter a value "1" (for Enabled) or "0" (disabled).

NOTE: Cheats will only work after a new level is loaded (loading savegame does NOT count).

NOTE 2: Don't use Load as it doesn't always work properly (go back to the main menu instead), but Save should be fine, except you don't get a save-point icon and the game will be called "TestGameName".