If you missed it over the weekend, Microsoft has finally released the PC demo for Forza Horizon 3. Here also are the PC requirements for this demo

PC gamers will be able to download the demo for Forza Horizon 3 via Windows Store (search for Forza Horizon 3). 

Minimum

    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i5 3570 @ 3.4GHz
    GPU: NVIDIA GT 750Ti or AMD R7 250X
    VRAM: 2 GB
    RAM: 8 GB
    Storage: 21 GB available space
    Expected Performance: 720p/30fps

Recommended

    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i7 3820 @ 3.6GHz
    GPU: NVIDIA GTX 970, GTX 1060 or AMD R9 290X, RX 480
    VRAM: 4 GB
    RAM: 12 GB
    Storage: 21 GB available space
    Expected Performance: 1080p/30fps

Ideal

    OS: Windows Anniversary Edition 64-bit 14393.102
    CPU: i7 6700 @ 4.0GHz
    GPU: NVIDIA GTX 980Ti, GTX 1080 or AMD Fury X
    VRAM: 6 GB
    RAM: 16 GB
    Storage: 21 GB available space
    Expected Performance: 4K/30fps