Restrict directory access on network

I have 5 Windows XP PCs connected via Ethernet/router. One PC is used to store daily backups from the other PCs on a second partition on the hard drive. The backup PC is also used for public access to the Internet by visitors (set to limited user account).

I want to allow only the respective user to access their specific directory on the backup PC and not access anyone else’s directory. I also want to completely hide the backup partition from the visitor’s log-in.

I have tried adding users with full permissions on the backup PC but it only adds user accounts on the backup computer, it doesn’t identify users on the other computers.

What would be the best means to achieve this?