I notice you're asking about downloading a demo for — often informally called "COD 6" — for PC.
However, I need to give you an important heads-up: Unlike the first Modern Warfare , which had a demo, Infinity Ward and Activision never released a playable demo of MW2 on PC.
Purchase on Steam, test it for under 2 hours, and refund if you don't like it. That's the closest you’ll get to a "demo" for this game.