XP logon to NT4 deletes user profiles

Our NT4 network has 100 client machines (Win98 and Win95) used by 300 students and staff. Each user has their own logon and has a mapped home directory on the server. We are about to install a lab of WinXP clients. As a trial, I logged on to a student account from one XP client. I discovered that the user profile on the server was overwritten and all the student’s work disappeared. [Yes, I had backed up first ;)] How can I set up up the XP machines so that when they logon to the network, existing student data in their mapped H: drive is preserved?