I have been using a Lexmark 901 printer with windows for a long time, and decided recently to install it to LTS 10.04. The driver was easy to find on the Lexmark site, but as soon as I extracted it and tried to install it, I got the root password needed message. I searched for information about that, and just about every search I could to figure out how to install this printer driver.
Here is what has worked for me on three computers so far...
Unplug the printer from your computer.
Download and extract the printer driver.
Open a terminal window and type in sudo nautilus
enter your password when asked, and you should get to a file manager window.
Find the icon you extracted from the driver download. It will probably be in filesystem, home, YOUR SCREEN NAME, downloads, or wherever you put it.
Click on the icon for the extracted file.
That's it. The Lexmark printer driver will install and guide you through the setup.