If you ever get a message “Unable to open TWAIN source” while trying to use your scanner, and you’ve reinstalled scanner software and/or drivers without any luck, check to make sure that Still Image Service is running and Startup Type is set to Automatic. I just spent an hour troubleshooting this problem with my Canon LiDE30 and it turned out that all I had to do is start the service.
Here’s another thing to try: add C:\WINNT\twain_32\CNQL20 to your system path (CNQL20 folder name may vary depending on your scanner’s model – browse the files to check)
The Still Image Service fix applies only to Windows 2000. To edit system path, right-click My Computer icon, select Properties, click on Advanced tab and click Environment Variables. Double-click on PATH under System Variables, append a semicolon and then the path you want to add (e.g. ;C:\WINNT\twain_32\CNQL20).