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I bought it from gamersgate last time for $5.99.
It is a Games for Windows Live title, you have to download the game from them, use the cdkey and login to GFWL.
I would suggest playing the 1989 NES Batman, it is very hard but worth it. Using an emulator helps as well. But it is better then the QTE style combat of Batman:AA as well as the auto-everything-ness of it that involves in skill and the horrible boss fights.