Let’s address the elephant in the room. You cannot download a native Windows .exe file of Forza Horizon 1.
Microsoft never compiled this game for x86 PC architecture. Consequently, 99% of links claiming "Forza Horizon 1 Highly Compressed PC Game Portable Download" are actually one of three things: forza horizon 1 highly compressed pc game portable download
If you find a "portable" version that works, it will contain: Let’s address the elephant in the room
Forza Horizon 1 Portable/
├── Xenia.exe (Emulator)
├── license.txt
├── FH1_Launcher.bat
└── game/
├── forza_horizon.iso (Typically compressed from 8GB to 2GB via Repack)
└── shader_cache/