The only Halo games to be released on PC were Halo: Combat Evolved and Halo 2, though Russians are enjoying a form of it called Halo Online, which was in closed beta back in March. It seems fans are tired of waiting for Microsoft to port a new Halo to PC, so they're making their own called Installation 01.
The developers are made up of programmers scattered around the world, but they have a unified purpose: bring some Halo-style multiplayer to the Windows masses.
Naturally, they're very cognizant of legal issues, and are abiding by the copyright laws. “We are abiding by Microsoft’s Fair Use Policy and their Game Content Usage Rules. Everything in the game is fan created and can be legally downloaded and distributed in accordance with the Game Content Usage Rules. We are not out to make any money, even to cover costs," reads the boilerplate on the YouTube video. In short, it's a labor of love.
The group has an official Facebook, and released a teaser showing off what to expect in the game, which is basically a fan amalgam of Halo 1 through 3 multiplayer. It's in pre-alpha, so it's pretty rough, lacking smooth animation or advanced physics, but it gives an idea of what they're aiming for.