: A free standalone expansion for the original PC port (Halo PC) was released years ago to support user-made maps. While you technically need an original CD key to install it, it remains a popular way for the community to keep the original engine alive.
While the original (2001) is not officially available as a free download, there are several legitimate ways to play the game or its modernized versions today. Official Ways to Play halo-combat-evolved-free-download-pc-game
: The modern standard for playing Halo CE on PC. It features remastered graphics, high frame rates, and is available for purchase on Steam or the Microsoft Store. : A free standalone expansion for the original