I didn't know there was a Windows Live... surely that is just the internet, lol
And no, Halo 2 isn't cross platform. Games need to be specifically made to be able to be played using different platforms. Like when GoW comes out on PC, it will not be multiplayer with the xbox community cause the xbox game wasn't made to work with PC. Unless the devs go insane and try to make everyone download a huge patch for GoW on xbox and enable it on the PC version, but for some reason I don't think that is going to happen, i dont know why, just a creeping sensation i get when i hear something RIDICULOUS.