I played this game in 2005 and there was no invincibility cheat at that time. When was the last time you checked? If it was recently, I believe you looked at the wrong game. Due to this game's age and lack of life, I doubt any changes have occurred to it recently, such as edits in the cheats.
An invincibility cheat might have been fun to mess around with if unnecessary. Hm, all this discussion has made me miss this game. I'll probably make a new file and play through it again. Thanks for the discussion.