How to require login to access shared folder using Windows XP Pro on peer t

I have 8 computers in a small business office using peer-peer networking, all PCs are loaded with Windows XP Pro. All computers have their own shared folder on one computer named Backup. There is a folder on the Backup computer that I want to access from any of the computers. I’ve mapped the shared folder on each computer on the network, but I want to require a login whenever this shared folder is accessed from any of the other computers so only users that know the username/password can access it.

I’m not sure how to accomplish this. Any ideas?