This is the classic "Gearbox" version. It is no longer sold digitally on mainstream platforms like Steam or GOG. To play this legally, you typically need a physical retail disc and a valid CD key.
Part of the Master Chief Collection (MCC), this version features remastered graphics and is fully compatible with Windows 7. It is the easiest version to acquire and install today via Steam . How to Download and Install Option A: The Modern Route (Recommended)
Here is everything you need to know about getting Halo running on your Windows 7 machine.
If you have a physical disc or have found a digital archive of the original game:
On Steam, it is listed as "DLC" for the Master Chief Collection , but you can buy just the Halo: CE Anniversary module for $9.99 without needing the full $40 bundle. Option B: The Classic Route