Unverified. Version unknown. This involves editing the game's ini file. Back it up before you change it or die.

Locate the hitmanbloodmoney.ini and open it using Notepad. Add the string EnableCheats at the bottom of the file and save the file as an INI file.

When you are playing Hitman, press C on the keyboard to access the cheat menu. The following codes may be entered:

Show OSD 1 or 0 .... HUD toggle.
Invisible Mode 1 or 0 .... Toggle AI Sense Player.
God Mode 1 or 0 ... Agent 47 takes no damage.
InfAmmo ... Set ammo to integer value. Unknown if integer will affect code.
InfClip .... Set magazine capacity to limitless.
Test Cloth ... Randomly changes player clothing to random skin in set stage. Complete Level .... Clears current mission.
Time Multiplier .... Time compression/expansion.
Teleport ... Random player spawn.
Beam here ... Spawn player at selected area on map (map open?).
Unlockable Real Ending
The ending where 47 dies at the end of the game (on Requiem) is not the game's real ending. What you need to do to achieve the real ending is to move the left joystick (it may be mouse or keypad on PC) back and forth (during the credits) until a surprise occurs.

Note that you will need to quickly eliminate all the witnesses in the level. All the guards in the main chapel carry weapons so you should take them out first. Good luck!