Have you noticed how you cannot buy a digital copy of Halo 2? It is because the developers called Bungie have released all rights and have abandoned the PC software for Halo 2. It is now possible to download both the Halo 2 trailer and the game and most users do not have to pay for the activation key to watch the trailer or play the game.
Watch The Trailer To See How The Game Plays
You should consider downloading the Halo 2 trailer to see what the game looks like. Halo 2 is rather old and doesn't look nearly as good as modern shooting games. The graphics are anemic compared to the rich textures of modern Bungie games and the controls are not nearly as sophisticated as the other Halo sequels. The trailer shows you fast-paced action allows you to hear the crisp audio and gives you a good impression of the overall game quality.
Try The Trailer Before You Try The Game
How is it possible that Halo 2 may be downloadable and/or that it may be played for free? The reason is that it is abandonware where the original creator has no interest in making money from it or enforcing copyright. If you remember the old Halo 2 game from 2004 then consider downloading the Halo 2 trailer and maybe trying the game.