From the GTA IV readme:
"During the Grand Theft Auto IV installation you will be prompted to register with Games for Windows - LIVE and Rockstar Games Social Club. Please note that registration into either is not mandatory to play the single player campaign. In order to experience the maximum functionality of Grand Theft Auto IV PC, including the ability to play multiplayer, share your own user-created videos online, and compete and share stats with friends and other gamers, we highly recommend that you sign up for both."