I used this method and managed to install halo 2 vista on server 2008 and even got it to work online.
1) Insert the Halo 2 DVD
2) Run Startup.exe on the Halo 2 DVD
3) Install the game
4) Install XLiveRedist1.0.6027.msi and vcredist.msi from the redists dir on the Halo 2 DVD.
5) Copy xinput9_1_0.dll to system32 for the game to work.