is no longer sold and its servers (Games for Windows Live) are dead, the community created Project Cartographer What it is : A mod for the original Halo 2 Vista

The Microsoft Store offers Halo 2 as part of the Halo: The Master Chief Collection. This collection includes all the Halo games, including Halo 2. While it's not entirely free, you can download the collection for a relatively low price.

If you are searching for you are likely facing two harsh realities: The original 2007 Windows Vista port is notoriously broken on modern hardware (Windows 10/11), and legitimate free copies are rare.