I'm not sure who deserves recognition on this one, the wonky video card; its creator or manufacturer, or Cannon for its software or scanner.
I recently installed a second video card into my computer. It was acting weird and at times it was crashing. When it was working it felt like the machine was running slower. I lived with it and got it somewhat stable, but still things felt slow. I was willing to live with that for a while in order to have a dual monitor setup until I could do something else.
I have been scanning in a bunch of documents and the pile was almost gone. I loaded up the Canonscan Toolbar version 5 and chose PDF as I have been doing for hundreds of documents. The PDF scan box came up and I clicked scan. It gave me this error box that stated:
Unable to open TWAIN source
Please check connection
Then re-start Toolbox
I tried unplugging the power and plugging the scanner back in. I tried unplugging the USB cable from the scanner and plugging it back in. I tried rebooting. I tried uninstalling and reinstalling the Canoscan Toolbox. I tried hacking registry keys. I had even at one point tried to fix I am going to explain below and that didn't work either at that time. I tried apply all of the updates that seemed reasonable from Windows Update. I tried scanning with Windows Live Photo Galery and that scanned fine. I removed the offending video card. I loaded a virtual machine with VMWare Player and it scanned fine with the Canoscan Toolbox software installed in it. The only time it didn't work was when the host's Windows could not even install the driver, but I got around that somehow.
I ended up finding that some had found it helpful to change the windows path variable. I have been running Canoscan Toolbox with this Cannon 4400F scanner for years and never had to hack the path variable, but I tried it anyway. As I said I did it twice during the process and it failed to be of help the first time. After installing and unstalling it got removed from the path again.
The path that needed to be added to the Windows path variable was C:\Windows\twain_32\CNQ4803. I have heard that the last folder in this path is specific to each scanner. This one is for a Cannon 4400F.
After that it began working again.