The issue with TWAIN on Windows Vista (and Windows 7) is that the TWAIN interface is wrapped by the new Vista WIA methods. With the printer and the software, they work in a different world than Vista. So, one way to make this work is to put Vista into a compatibility mode. Here is what I would do:
1. Update the CanoScan LiDE 20 driver and your software, specifically for Vista. The latest update was 2007 and can be found here:
Test this first, or verify that you have the latest driver.
2. Locate the executable for ScanGear CS 7.0 in the program files folder on your Vista computer. Right-click the executable and select Properties. Then select the Compatibility tab. Click the check box and select Windows XP (Service Pack 2). See the screen print here:
Please try the above steps and let me know if this has helped you or if you have any other questions. I can give you links to further research this issue for Vista and backwards compatibility if you need it for other applications. Also, as a quick step, you might want to perform step 2 and test it out first. I always reboot after making that selection, then test it. It's a "Windows thing".
So, you have the latest versions of software and driver installed for the printer, and you put Windows Vista in compatibility mode by changing the settings on the ScanGear executable?
If those didn't work, I have some other things to try. Is your Windows Vista a 64-bit system or not? Do you have more than one scanner installed?
In the meantime, I'll put together some other possible solutions (I don't have that scanner so I can't test) based on my knowledge of how Twain works on Windows.
I'm still checking on some other areas. In the meantime, let's check to see that the Windows Image Acquisition service is running.
1. Open Control Panel | Administrative Tools | Services
2. Scroll down and find the Windows Image Acquisition service.
3. Ensure that it is "started" and is set to "automatic", by double-clicking the entry and using the Start button if the service is not started, and use the startup type dropdown to select Automatic.
Let me know what the setting was.
Are you always logged into your PC as the administrator on Windows? Do you recall installing the software and driver as an administrator?
Another question, did you upgrade to Vista from Windows XP? If so, you need to uninstall all Canon software and drivers, basically removing them from your system, and reboot. Then go to that driver/software download link I gave you and perform the installation of the scanner, again.
There is no doubt that the Canon scanner and software works on Windows XP. There are some peculiarities in the way Vista handles image capture, so you son should be able to follow the links I posted to get the gist of it.
Canon does support that scanner on Vista. The wording they post about "other applications" shouldn't apply unless you are using Word or some other software to try and capture the image. Directly from ScanGear it should work fine. The Canon link offers a way to send an e-mail for help. That may be an option. We've gone over all the other basic troubleshooting that would apply in this exercise.