GTA 4 needs tweakin on a lots of machines,
The best way I have found to get the game to run is by creating a file a called comandiline.TXT in the games directory and playing around with the parimeters,
The parimeters can be found herehttp://www.gtagaming.com/forums/showthread.php?t=111441
Just copy a few random ones like "-norestrictions" into the text file and save it as commandline.txt in the games directory.
It should work ok once you'ved played around with the settings.