well considering they dont even have a definite release date, I'm not sure why you are asking... I know xbox magezine typically puts out demos for games but I doubt it will be until a ways after the game is out.
To my knowledge its not going to be a PC/xbox release neways, at least when it comes out. All ive ever heard of is the xbox release so I wouldnt count on anything you can download for a really long time
Halo on PC came out a lil while after came out on xbox (and im sure you've heard all the criticism about it - bad fps etc). I would love halo 2 on pc but i think it wont come out until at least 4th quarter. But a more realistic date would be 2nd quarter 2005.
There will not be a Halo2 demo. As said before they havent said if its going to hit the pc. They have been running a 1000 person test over a secure line w/ in M$ for the xbox version. Anyway you dont need a demo for this game, as soon as it hits you should buy it.
im sure the wont be able to help themselves. I mean the game should be a big hit. at least with xbox owners. It is just a matter of time before it is released on the pc.