Halo 2 for windows is the buggiest, most problem laden experience of your life if you're tying to playing on Win7. There is no optimal set of hardware/software to play it with, because it has DirectX problems, and everyone needs DirectX 9 to play it. And if you're on a semi-new Nvidia card, well Halo 2 hates Optimus, so unless you can turn it off in BIOS you're double screwed. And even if you can avoid both problems above, Halo 2 has, has to be the first Games for Windows Live game that you install, Halo 2 doesn't like it if you already have GFWL installed. So even if you could get a server running, you would not get that many able to play it due to above issues.
Halo CE however, has literally none of the issues above and has not had problems that haven't been patched. True it doesn't have as nice a graphics engine, but who gives a shit when you can install and play it with no problems on any OS that anyone would be using. (Even Linux?)