Context:
In the first corner: A Mac OS X Leopard (10.5.3) machine
In the second corner: A Kyocera 2000D laser printer connected to a Windows XP printserver which is protected by password and username.

Problem:
Adding the printer via the standard Apple-way as a windows printer works – but printing does not work because it is not possible to authenticate against the printserver.

Solution:
You need to add the shared printer via the hidden “Advanced” printer feature in the Print & Fax preferences.

• Open “Print & Fax” in “System Preferences”
• Click “+” to add a new printer
• Control-click (or right-click) on the toolbar at the top of the window and select “Customize Toolbar…”
• Drag the “Advanced” icon to the toolbar and select “Done”
• Click on the “Advanced” icon … and be patient … after a moment or two, you can select “Windows” from the “Type” dropdown-menu
• Enter the SAMBA URL of the shared windows printer in the “URL” textfield:
  smb://USERNAME:PASSWORD@WINDOWSDOMAINNAME/PRINTSERVER/PRINTERSHARENAME or
  smb://USERNAME:PASSWORD@IPADDRESS/PRINTERSHARENAME
  If the shared printer does not need a password, you can use smb://USERNAME@…
• Fill out the other fields “Name” and “Location” as you prefer
• When printing the first time you will get prompted for your USERNAME and PASSWORD. Fill in your windows printserver username and password and enable the “Add this to your keychain” checkbox.

… it’s nearly plug-and-play