At first disconnect all other USB devices (except mouse and keyboard) and try again. Click on the unknown device in device manger (be sure that it is the scanner) and update the driver manually. 1. Mark the mentioned device (marked with exclamation/question mark) and click with the right mouse button and choose “properties” 2. Choose “update driver”. 3. Choose the recommended “install driver automatically” option. 4. When the driver location will be asked for choose the CD-ROM drive and insert the software CD-ROM of the scanner. The missing .inf file is on the software CD. This problem might not occur, if the software was installed before the scanner is connected.