The Microsoft Xbox Store has a digital download option for "Halo 5 Guardians," for Xbox One. As of 2015, Microsoft have no plans to bring the main "Halo" series to PC Steam, according to CinemaBlend. There are also no digital download options for the PC version of "Halo: Combat Evolved."
The latest game in the "Halo" series, "Halo 5 Guardians," is not available for PC. However, Microsoft does sell the smaller "Halo: Spartan Assault" and "Halo: Spartan Strike" games via the Windows Store and Steam.
The trial version of the original PC "Halo: Combat Evolved" game is available for download from CNet.com. However, the full version is only sold in physical, boxed form.