Modern Warfare 2 won't launch if the game launcher like Steam or Battle you use for this game is run without administrative privileges. Hence, to solve this issue, you can try reopening the specific game launcher as an administrator. Step 1: Right-click the shortcut of your game launcher and select Properties.
Because Modern Warfare is published by Activision, which is a part of the bigger Activision/Blizzard company. So to save money, they just put it out on the Blizzard launcher (Battle . net) instead of having to pay Steam to sell it here and they keep a part of the profits.