The actual installation of Modern Warfare II demands well over 100 GB of storage space. If the .rar file you are looking at is only a few gigabytes or labeled as "highly compressed," it is fake.
Activision's official launcher handles automated security and file verification. call-of-duty-modern-warfare-ii.rar
To experience the campaign or multiplayer safely without destroying your PC, stick to verified and authorized distribution platforms: The actual installation of Modern Warfare II demands