Rockstar has released a FAQ regarding the copy protection that will be used in the PC version of GTA 4, here are the details

    * Activation via Internet (1x per system, no limits, Fallback for systems without web access)
    * Copy protection Securom: The original disc is required for playing (Execpt: Download version of GTA 4, for example from Direct 2 Drive)
    * Requirements for playing (singleplayer): Games for Windows Live account for saving/achievements (1x free creation), original disc, Securom runs in the background
    * Requirements for playing (multiplayer): Active internet connection, Games for Windows Live account, For uploading videos: Rockstar Social Club Account.
    *
    * If you don't have web access or don't want to connect your gaming PC to the Internet, Rockstar wants to offer a not yet specified system to activate GTA 4 via another PC with web access. Details are supposed to be available on launch.
    * For the normal GTA 4 (PC) version with disc installation, activations are unlimited. But the original disc is required for playing.
    * This does not apply for legally downloaded versions of GTA 4: Depending on the vendor the following is possible: Five parallel installations with the possibility to revive a "free” installation, if an old installation is deleted. For this purpose the customer has to have web access with the PC GTA 4 is installed on.
    * Those playing GTA 4 with an illegally hacked version have, according to Rockstar, to fear that the game will not be fully playable.
    * If you change two major components of your PC (like the graphics card or the CPU), you will possible have to reactivate GTA 4.

Phew that was a mouthful I can tell you, whilst some will find it disappointing that GTA 4 uses such an invasive DRM system it does seem as if Rockstar has tried to weigh the need to protect their work against the inconvenience to the customer and for the boxed retail version there are no activation or install limits.