If you downloaded a "repack" or non-official version of the game, the file is often modified and easily flagged by Windows Defender. The Safe Solution: Verify Integrity of Game Files
The next day, Jack decided to try again to get Call of Duty: Modern Warfare 2 working. This time, he went straight to the official Steam website and downloaded the game from there. It worked perfectly, and he was able to enjoy the game without any issues. Call Of Duty Modern Warfare 2 Steam-api Dll Download