#68, you don't really NEED to upgrage the other computers to XP, but it'd make things easier on you. $180 easier, NO.
IF you are still in school, or know someone who is, MOST universities sell copies of WinXP (professional edition, no less) for FIVE WHOLE DOLLARS.
The Uni versions come with SLIGHTLY fewer bells and whistles....the installer, for example, doesn't have the cute pictures of Beaver Cleaver and his mom pretending to play a game. But it does come with all the networking options (as opposed to Home edition), and is, happily, 100% legit.
At $5 a pop, you could afford to upgrade all your machines. I'd hope. Some Unis only allow one copy per student...a student buying 20 copies of WinXP just doesn't happen. But hey, seriously, you could sit on a college campus and pay people $5 to buy you a copy...effectively putting you at $30. It wouldn't take very long to convince 3 students to do this for you; there's plenty of kids who just arrived there with a computer pre-loaded with winXP...they have no use for another copy. And as any college kid knows, $5 gets you dinner.