The first flighting (Microsoft for a Beta release) of Halo: Combat Evolved Anniversary for PC is officially underway, with a small number of insiders now giving the game it's first test run, for a limited period of time at least.
It'll be the next game in the Master Chief Collection, a project to update and bring various games in the Halo series to PC, starting with Halo Reach, which is available now. Members of the Halo Insider Program are being invited, albeit in small numbers, so if you're part of the program, check your inbox, you might be one of the lucky few chosen.
The test version only contains four campaign missions and multiplayer, including custom game modes, so it's fairly inclusive, although not fully complete.
If you're not an insider, you're out of luck, but you can still sign up to the program in the hopes of being chosen for future tests on the official Halo website here.