Dont Listen to the BS Being Spouted here... I Have Installed and Reinstalled this game Almost 20 times. It will have proper keys if you Buy it from AMAZON and it can be reinstalled as many times as you need. (
http://www.amazon.com/Halo-2-PC/dp/B000NV8F58/ref=sr_1_1?ie=...)
(Seriously.. 20 bucks is to much for you?!?!?!? Just don't order Dominoes this week.... Halo 2 is worth WAY more than 1 Pizza!)
Just don't share keys and make sure you run the update tool from the games for windows/ halo 2 folder before you go on line... and once you start the game, YES you want to update live as many times as it asks for... until its done. It takes about 10 to 15 updates before it's ready to play and lets you sign in.
And Multiplayer achievements ARE Available on live, but you have to to it LANPARTY style.... eg: 2 computers or more over a network and be playing the classic rules (no game or game type Mods!)