The file system shouldn't matter. As long as the application isn't designed to do low-level stuff, yes, the application, if designed for 2000 should work on XP probably 99.9% of the time. Meaning 1 app in 1000 won't. Those that might have issues are things like older antivirus engines that might have problems with some of the relatively minor changes/tweaks in XP and other programs that interact with the core of the OS more closely than, say, a word processor or database app.