It seems, as if Halo 2 is no longer a Windows Vista exclusive title. Various sources confirm, what appears to be a solution for running Halo 2 on Windows XP machines. A tool called Halo.2.XP.Patch-NoGrp circulating the internet seems to enable owners of the game to play it without having DirectX 10 or Vista installed.

The process appears to be rather trivial, since the readme.txt quoted on several file sharing websites states only 7 steps before the game should start up. In order to make it work, you'll need the tool, a Halo 2 DVD and be able to do this:

Installation Instructions:
1) Extract the archive.
2) Insert the Halo 2 DVD
3) Run Startup.exe on the Halo 2 DVD using the included Loader.exe
Ie.: Loader.exe D:\Startup.exe
4) Install the game.
5) Install XLiveRedist1.0.6027.msi and vcredist.msi from the redists dir on the Halo 2 DVD.
6) Copy the following files into the directory you installed Halo 2:
dwmapi.dll, mf.dll, MFPlat.dll
7) Finished. To play the game, run halo2.exe using the included Loader.exe

Still, a number of people report the game to crash after several minutes and for some it doesn't work at all.