Gears of war problem PC

I just finished installing GoW on my laptop. when i click on the game launcher it gives me an error saying "you cannot run the game with modified executable code. please reinstall the game." so i reinstalled the game and i get the same error message. i haven't reconfigured anything nor i modified anything. what's the solution for this problem?

my laptop description:
amd sempron(tm) processor ssi-42 (2.1ghz)
3gb ddr2 system memory (2 dimm)
nvidia geforce8200m gw/5-in-1medial reader