Hi there. I've found a Trainer for GTA IV that came out very recently. It has a Forced Invincible mode that makes Niko into Superman, even between and during missions. It also has some good extras. I've been testing it on Vista, and it is very stable so far.
http://www.gtagarage.com/mods/show.php?id=5231
I started testing it in Alpha phase. It's now in Beta.
