XSane is a graphical scanning frontend that makes easy to use you scanner. But when i lauched XSane for the first time under my Debian Lenny enviroment, XSane reported:

Failed to open device `v4l:/dev/video0': Invalid argument

To prevent this error, you should comment out the “/dev/video0″ line in /etc/sane.d/v4l.conf. Now you should be able attach your USB scanner and launch correctly the program.
But what happens if XSane says that no scanner is found? Follow these steps that helped me to solve the problem:ù

1. Ensure the that libsane-extras package is insalled in your machine
2. Try to run XSane as root. If it work, and finds your scanner, you should fix the permeission sets on your scanner device.
3. Find out which bus your usb scanner resides on, this is easily done with the lsusb command:
   
   debian:/home/myo# lsusb
   Bus 008 Device 004: ID 054c:0377 Sony Corp.
   Bus 008 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
   Bus 007 Device 002: ID 05ca:183d Ricoh Co., Ltd
   Bus 007 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
   Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 003 Device 007: ID 04b8:010b Seiko Epson Corp. Perfection 1240
   Bus 003 Device 006: ID 046d:c01d Logitech, Inc. MX510 Optical Mouse
   Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
   
   You can see the scanner resides on Bus 003 and is the 007 device in this case.
   Change the permissions using:
   debian:/home/myo# chmod a+w /dev/bus/usb/003/007 .  Replace the bus and the devices numbers with yours.