Setting up Halo Custom Edition
Getting a CD Key
Well, I wont give you one but Google will. ;) Yes its legal as long as you have a full and complete copy of the game installed with legitimate CD Key (which those keys are.)
CD Keys have unlimited uses, there is only one catch... Some of the keys you find on Google (or Google Images) will have already been banned from some public servers.
Why use keys found off google? Well there is no point in trying to buy an original copy of the game past 2016 as Microsoft stopped generating keys for the game. As well as all of the keys for sale elsewhere are potentially already in use and being resold multiple times due to the fact that keys have unlimited uses.
Installing the game.
When installing the game make sure to uncheck the box that says Install GameSpy Arcade.
Also make sure to install the 1.10 update linked above after you have finished installing the game, it is required for online play.
Other things to install.
The following highlighted runtimes are required to play the game. The rest are optional but recomended as there are mods that will use them.