I have GTA IV for PC and I installed everything it needs in the installation process, but when I try and launch the game, I get an error code called GTA IV FATAL ERROR DD3D50.

Replace the disk...files might not be complete in the disk. I hope this
might help you.

Hey, Reinstall the game and if that does not work then try to install a different game to make sure it isnt you cd-drive. So if it isnt then it must be you disc. So best bet it to take it to your nearest game shop and get it cleaned for a few quid or replace the disc this is all i can suggest..
Sorry if this doesn't help..
Best of luck and thank you for using fixya.