Ok, for the people who are still having problems with GTA IV not loading after pressing "Start" in the main menu, here's REALLY working fix I have found. Well, there was a fix around the internet with a offline Account in Windows Live, but what if you want to play online?

Here's what I do, so I am able not only to play Singleplayer, but also Multiplayer. Its really simple.

• Start GTA IV as you always do, don't log out anywhere (GFWL, Social Club)
• Open up GFWL and log out
• Press the "Start" Button in the main menu (you will be warned that you can't save without being logged in. Ignore that.)
• Now the game should load without problems and a new game should start
• Skip the intro by pressing "Space"
• Open up GFWL
• Login to your GFWL-Account, NOT an offline Account (otherwise there's no online gaming available)
• Press escape and load your normal savegame
• Play!

This has to be done EVERY time. I know it's shit, but it's the only way known to me to avoid this problem.