Modders have managed to obtain a version of the planned-for-Russia, free-to-play Halo Online and have captured video of the game in action.
Halo Online is built using a modified Halo 3 engine, so many consider this to be as close to a PC version of Halo 3 that will ever exist. Some major differences from Halo 3 include a new setting, (the UNSC space station Anvil), sprint capabilities, and a wider collection of on-screen displays. Halo Online is also multiplayer only, so you won't find a campaign.
Microsoft has been actively pulling down videos like this showing off the game, so there is a good chance the video will disappear from YouTube.
[Source: GamingPranks on YouTube, via Reddit, Polygon]
Our Take
Building a Halo experience exclusively for Russia is an odd choice for Microsoft, especially considering China is the land of free-to-play online shooters. I hope that the game eventually officially makes its way to North America at some point.