*pats both on backs* I sympathize. Allow me to explain:
At my college, I use Macs with OS X, but at home, my bro's PC uses ME(No jokes, he can't afford XP, goons,), and my Mom's uses XP. I need to partition my external 40 GB HD so that when I'm using it at school, I can use 20GB of it at the school on OS X, but the other half is usable only to Windows-based OS's.
Hope that helps.