You'll need to stop using simple file sharing. From any Explorer window, select Folder Options from the Tools menu, and then click the View tab. Turn off the Simple File Sharing option. Then, create users on the XP machine sharing out files, give those users appropriate passwords, and then assign rights to those users for the files and/or folders you want to secure. Once that's complete, when someone wants to retrieve a protected file, they'll be prompted to enter a valid username and password.
