Hmmmm... I am running Halo 2 on XP pro SP2 - with no problems whatsoever
However - so far I have only played the first level... :/
I will supply you with some more information tomorrow as I no longer remember what I did to make it work anymore.
I would love to research it now but it's almost 3:00am here and I need to get some zZZzzzzZZzzzz's... Sorry!
Edit: If your considering moving to Vista, first ask yourself: "Do I find Windoze XP easy to yous, and does it do everything I want/need it to do"? If the answer is yes.. well - what do you think that means?
Also, another thing to keep in mind is that Vista carries on the normal Windoze trend... This meaning that it takes more recourses to run pretty much any program. Or, in very simple language - on average stuff a tiny bit runs slower on Vista compared to XP.
Here is a very good link that shows the performance difference from XP to VISTA
http://www.tomshardware.com/2007/01/29/xp-vs-vista/One last note - if you want to get Vista because it looks really sleek, cool and easy to use - there are pleanty of ways to make XP look very similar. Eg.
http://bronch.com/downloads/risherz/vista2.0.gif*I am not a Vista owner - I have only used vista for less than 10 mins myself, so I am not in any way an expert in this feild.*