GTA4 is a very unstable game, even on the consoles it preforms poorly, and there are noticeable just in the source.  So that being said, even though the game is very glitchy the quality of the game over the console versions is vastly improved, from the ability to control your graphics to be vastly superior to the console versions, to the custom music you may place in the game, to the other little things that the computer version has that the console does not, like clothes selection and such perks.  As a game, they did a good job making you feel like a off the boat thug, the story is amazingly entertaining and of course you can just not do anything and drive down the sidewalks painting them red from the blood of the unsuspecting, damn you just hit a cop and got two stars, time to run away, damn killed another cop and got 3, crap a helicopter - where's the RPG?  One thing that really annoyed me about this game, was Rockstar's online client, For some reason Rockstar feels cool enough to add an application to your windows startup and sign you online so you can play multiplayer or save your game.  Oh yea, if you don't sign up with rockstar and run the client, then you can't play any multiplayer and good luck saving your game.
All in all, too complex for the common person to try to get it to work on a Windows 7 machine 64bit, took me about 1 day of searching what to do, compatibility for Windows2000 seemed to work after getting all the files in ship shape and updating SDK visual basic versions and windowsLIVE,  but on Windows7 32bit worked without too much trouble.