I Bought Yesterday Crysis 2...i installed it, then i tried to Activate it...then it says Activating successfull. But the Game dont start...well then i had to Activate it again still dont start. I really dont know what to do.

Try boosting your GPU volts(use MSI AfterBurner for this) by 0.05, if that doesn't work you could try loosening your RAM timings.
C2 stresses a system like no other game.

Um..well i really dont know what u mean -.- u can maybe explain it for me?