1st: I own a legal copy of the game which I bought about a year ago
2nd: sorry for my english
3rd: I think I have a solution for the ZLIB20 Fatal Error (I'm still testing it and no crashing for now)
after months of reinstalling and everytime combining it with diferent patches and with different addon programs (like asi loader, SNTrainer and etc, because I love the custom addon features) I found after attending a mission that the game crashes in 100% of the time when you are at the airport with the ZLIB20 fata error. so I realized that every time i reinstalled the game 1st i changed the orig vehicles.ide,handling.dat,carcolor.dat and the vehicles.img with mine fully customized from the different sites files, so I reinstalled the game (again) loaded it with the custom programs and trainers, upgated it with the 18.104.22.168 patch and started it with the original vehicles, thye I went straight to the airport and the game doesn't crashed for a second, then I spent few days that way without any problem
After that I wanted to find why the game gets the ZLIB20 fatal error with the custom cars, because I hated the original ones, they are dull, simple and ugly and I love my downloaded custom ones
so I started removing them one by one to find the one that causes the error (it happened to me before with different error, so I thought that is the problem) and with every car i started the game and spawned them with the trainer and one time by accident I loaded the "airtug" and the game crashed immediately!
It was still the orig one, so I found on the net custom made one http://www.gtagaming.com/downloads/gta-iv/vehicle-mods/3192
and changed it!
then i started the game and straight to the airport, no crashes, custom cars, custom features and I love it! Now I play it with a big smile on my face
Hope I've been helpful