Thats not the idea. See, you need it on LIVE so you can show them off to everyone else.
The problem they are going to have with achievements and PC gamers is simple to see. If you want to raise your "gamerscore", you've gotta get some achievements. Thats easy to do on the 360 because you can RENT games, get the points, and be done with it. You can even rent the shitty games you'd never play and get the points to raise the score. But on the PC, you can't rent games. So no one is going to bother buying games for the achievements. Pirating them? Now thats a possibility....
Heres more halo 2 junk.
GamespotHowever, you will most definitely need Windows Vista, because Halo 2 will only run on Windows Vista PCs. According to Clowes, that's because the game takes advantage of many Vista-only features, such as rapid install. When you first insert the Halo 2 disc into your PC, you won't have to install the game like you traditionally would a PC game. Instead, like a console game, Halo 2 will begin to load up and you can play immediately while it installs in the background.
Ok, thats a cool feature. Not enough to be like "OMG buy Vista!"...but a cool feature anyway.
If you're currently an Xbox Live gold member, then you needn't worry about signing up or paying for a separate service, because we're told that the Xbox Live membership is universal with your Games for Windows Live membership. And if you don't have Xbox Live, signing up for Games for Windows Live will also set up your Xbox Live account. The two are basically one and the same.
Silver = free, Gold = Pay. They do mention that you DO NOT need Gold to play online. So...why pay for Gold? They didn't say.
They go on to mention the Halo 2 map editor. Lets just hope it isnt like the pile of shit map editor released for Halo 1.