Require Login to Access Shared Folders
If you share folders on your Windows PCs, you can require remote users on the network to login in order to access them:
Open the Run dialog or a Command Prompt, and then enter the following and hit Enter: control userpasswords2
On the dialog box that appears, check the Users must enter a user name and password to use this computer check box, and then click OK.
Remember if you have a simple Workgroup network without a Windows Server Domain, remote users must login with a Windows account on the PC hosting the shared folder.