Microsoft has published a short video about Halo: The Master Chief Collection that seems to hint at an early release of Halo: Combat Evolved Anniversary on PC. When exactly that will happen, Microsoft does not specify.
The video is supposed to be the background for the Combat Evolved Anniversary menu. As for the dialogue, it’s a reference to the introductory dialog of Cortana and Captain Keyes from the first Halo — hence the guess that Microsoft teases the next installment of The Master Chief Collection on PC.
The main feature of Combat Evolved Anniversary is significantly improved graphics. There will be a special button that allows you to switch between the old and new look on the run.