Okay, here's the deal. I had a original copy of GTA IV installed in my PC, but we needed some space in our hard-drive, so a uninstalled it after awile. Now I wanted to play again, but the game doesn't start, I click play at the Rockstar Social Club window, but nothing happens! I installed it, patched it, installed SP3, but nothing changed. It doesn't even show an error message! Can anyone help me here?

Uninstall it again, check your program folders and delete any 'Rockstar' or 'GTA IV' folders. Then reinstall. That's all I can think of.

I tried it and it did not work. Sorry.

Are you sure its not DRM related? I'm not sure if GTA IV had much in the way of DRM on the PC version as I bought it for PS3, but you might want to try a noCD crack type of thing.

But doesn't DRM prevents you from installing? Cause it is installed, I just can't play!

Depends on the DRM and your PC setup. I've heard of some DRM finding 2 CD/DVD drives and not letting you play the game, or even detecting IMG burning software and kicking in that way. Its a long shot but its worth a check if the crack will fix it.