Note: I cannot provide direct links to ROMs, as they exist in a legal grey area. However, if you own the game, creating a backup for use in an emulator is generally considered fair use.
The websites that pop up when searching for a highly compressed PC download are riddled with dangers: Forza Horizon 1 Pc Download Highly Compressed
To play Forza Horizon 1 via emulation, your PC needs to be significantly more powerful than an actual Xbox 360. Emulation is demanding. Here are the general specifications recommended by the community to run the game smoothly via Xenia Canary: Note: I cannot provide direct links to ROMs,
If emulation sounds too complicated or your PC lacks the hardware power, you have other avenues to explore: Emulation is demanding
How to Actually Play Forza Horizon 1 on PC: Xbox 360 Emulation
To understand why a native download doesn't exist, it helps to look at the timeline of the franchise's platform availability: Game Title Original Release Year Official Platforms Xbox 360 only Forza Horizon 2 Xbox 360, Xbox One Forza Horizon 3 Xbox One, Windows PC Forza Horizon 4 Xbox One, Windows PC (Steam / Windows Store) Forza Horizon 5 Xbox One, Xbox Series X/S, Windows PC