1. Do the network communications between the PC's work as expected under normal circumstances? Can you send files in both directions between the 3 PC's and the dat drive PC without any issues?
2. Have you disabled the XP Firewall? If not, decide which Firewall you want - XP or ZoneAlarm - and then disable the other one.
3. Are you logged in as Administrator? A Standard User, as defined by XP, does not have sufficient priveledges to perform a Restore.
5. Do you have any encrypted folders for a user other than the user that is trying to perform the Backup? Even an Administrator cannot Restore an encrypted folder to another user account. (This makes sense otherwise encryption would be rather useless).