Dude, I got the same message. I got it to work but all of hacks were gone. You can do it but your hacks will be gone. First, Open up steam......Go to "My Games". Then right click on the icon that says "Call Of Duty: Modern Warfare 2 - Multiplayer." Click on "Properties".
There should be 4 tabs at the top. Click on the one that says, "Local Files". Then click on "Verify integrity of game cache" It should start a download that will make MW2 work again. But your hacks will be gone. Your profile hack will be there. Remember: Thank me if I helped.