Anyone still battling with this "Authentication Required" when connecting to a shared printer connected to a Windows 7 PC try what I did, sorted out the issue.
Open keychain access and delete all passwords/items linked to the problem printer.
Try send a print through again.
It should now give you a dialog box to enter a username and password.
Check remember in keychain.
This worked for me after trying everything else.
In the printer queue, where you see the "Hold for Authentication" message, just click the refresh icon. A dialog should appear where you an enter your network credentials.
I don't know why this worked, but it did. Perhaps will be helpful to you.