Yes. You can.
halo 2
not legally
This is kind of a two-part answer, because I'm not sure about the question. 1 - Halo (the original) will work on XP, and should work on Vista. 2 - Halo 2 will only work on Vista.
Halo 2 is made for Vista to get us customers switch to Vista instead of using XP.
Obviously, because Vista wasn't around at the time Halo 2 for PC was made, and this therefore makes Halo 2 incompatible for Vista
Halo: Combat Evolved runs just fine on Windows XP.
Yes sadly you really do need Vista to be able to play Halo 2. There are a few cracks out there that sort of make it work on XP but so far there's been no real or legit way to fully play Halo 2 on XP. Sorry
Yes, its instores to buy; A Windows Live game.
Only for Windows Vista, but you can use it on Windows XP, but that will stop Multiplayer from working.
yes
halo 3 odst, unlike halo 3, has no matchmaking, sorry
you get one xp per win