Its really easy, you just disable one drive, and install XP on it, then do the same for the other drive. Then put both drives into your system and you can control which drive to boot into via the BIOS.
So you don't want to reformat? Well you can always make a partition and just format that partition as your system file, but the its going to be troublesome. Why do you want to dual boot anyway, just stick with one operating system.
I love Halo 2.
I even bought an Xbox just to play Halo 2.
Now I moved up to a 360 and love it.
Actually, I went to GameStop today, to buy Halo 2 Vista, and they had a sale.
Buy two used games, get the third one free.
I bought FEAR,Call of Duty 3, and Project Gotham Racing. And I already own Crack Down and Gears of War.
Turning the tide, I happened to get vista, and for being loyal I should be the only one who can play halo2 (And by "I", I mean anyone with vista). The argument can go either way, just a little point of view.