I'm also running Windows 8 (Release Preview right now). I don't think you should have any problems running GTA IV on Windows 8. I can run old and new games on my PC.
Have you tried reinstalling Grand Theft Auto IV?
Also, my custom PC wouldn't run newer games due to outdated BIOS. Have you tried upgrading to a new BIOS on your motherboard?
If that doesn't help, contact support on whatever medium you purchased the game from.