Halo Single Player Version 3 (SPV3) is a rather wonderful fan made re-imagination of the original Halo Combat evolved. The mod is free but does require a valid key code from an original game to install.
Sadly only the first two Halo shooters made it to the PC along with a more recent RTS game called Halo Wars but I still love the series. SPV3 replicates all of the missions of the original game and extends them with extra sections, extra enemies, new weapons, new vehicles and new abilities. Did I mention the extra enemies? There are a lot of new enemies. Happily they game supplies you with a lot of extra firepower to deal with these enhanced threats.
The mod is incredibly polished and feels like a professional game. If you are interested you can grab the latest version from this Reddit thread: https://www.reddit.com/r/halospv3/