Thank you for the post.
"why does the PC name appear in the account name ?"
Please review the screenshot on the left below.
Edit > Preferences > General tab has an option for "User name:" and the value in this field will populate in the "Account Name" field when prompted for authentication.
"now both files require log in details on the server"
Please review the screenshot on the right below.
File > File Options… has a setting for "When opening this file - Log in using:" and new files are setup like the screenshot below by default.
If the box is unchecked, then the user will be prompted for an Account and Password.
Even if the box is unchecked, as long as the Admin account name and password were not changed, then Admin without a password will still log into the file with the default [FULL ACCESS] account.
Thank you so much - problem solved