Registry Hack:
NOTE: Changing your registry keys can lead to system or OS problems. Only change registry values if you know what you are doing or have a registry backup.

Edit your registry and find key:
HKLM/Software/Computer Artworks/TheThing/1.0

Right click and create the following new Strings and set each with a value of '1' for Enabled, '0' for disabled:

PlayerInvulnerable: Unlimited Health For You
NPCInvulnerable: Unlimited Health For Allied Friends
FullWeaponEquip: Get All Weapons
DoInGameLoadSave: Load/Save Game Appear in the Options Menu

NOTE: Cheats will only work after a new level is loaded (loading savegame does NOT count). Some people have reported problems with the LOAD function, so you should use the main menu for LOAD purposes.

Level Select:
Create a string value called "DoLevelSelect" and set the value to '1'.

Now, create 19 string values titled "LevelSelect#" where the number sign "#" is replaced with the numbers 1 through 19. Each of those string values will be assigned a value that corresponds with each level. To get these values, open the Game folder and go to data\levels. Put the name of each file (do not include the '.dll") you see there as the value for each of the LevelSelect# entries.