Rockstar talks GTA IV PC DRM
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Rockstar's outlined the DRM methods used for the PC version of GTA IV in this IGN article, confirming that you'll need to register the game every time you install it on a new machine.
The title will use SecuROM, but you'll be able to install it as many times as you like.
There's a lot of info through there, so if getting het up about anti-piracy measure is your thing, it's Christmas come early.