Bad scanner, bad driver, bad cable connection to the computer.
Are these thin horizontal lines across the path of the scanner light travel
at regular intervals or irregular intervals? Same or different colors?
Have you checked the glass for lines from someone using a marker on or
scratching the glass?
If they are in the same direction as the path of the light you have a "photo
diode pixel" failed.
Try the scanner on another computer (install the driver first before
plugging it in) to see if the problem goes away.
If under warranty, contact the scanner manufacturer for support.
