Where do I find a twain driver for the scanner?

I found that ArtoTyrväinen's solution also can be adapted to work for the CanoScan 8400F and Windows 8.1 Pro.

It had previously worked on Win 8.0 Pro so I was surprised that I had problems with 8.1 Pro. It took me some time to come across his solution.

Go to Canon for the device drivers  http://www.usa.canon.com/cusa/support/consumer/scanners/canoscan_series/canoscan_8400f#DriversAndSoftware

and get the Windows 7 (x64) drivers (No Win 8/8.1 drivers exist)

Go to your downloads folder and look for a folder labelled: 8400fvst6410231a_64en

Inside that folder find the file "SetupSG.exe"

Right click properties on "SetupSG.exe" and go to compatibility tab of that file and select both "Compatibility Mode for Windows 7" and "Run as Administrator". Click Apply and OK.

Then double-click "SetupSG.exe".

After, download and install the CanonScan Toolbox setup file from the Canon website:

                Run "cstbwin4932en.exe" by double-clicking it.

Then in "This PC"->System Properties->Advansed system settings->Environment Variables->

System variables->Path  Edit this line by add " ;C:\Windows\twain_32\CNQ8400" end of line (keep whatever line is currently there...just be sure that you include the ";" to separate it from anything already there.

Check if the path is right on your c: drive

Reboot the computer to complete installation.

I haveCanoScan LiDE25 and Win8.1

Install the device driver first:

                Run "lide25vst6411011aen.exe" by double-clicking it OR

                Open the folder "lide25vst6411011aen" then double-click "SetupSG.exe"

Install the CanonScan Toolbox:

                Run "cstbwin4932en.exe" by double-clicking it.

Then in "This PC"->System Properties->Advanset system settings->Environment Variables->

System variables->Path  Edit this line by add ";C:\Windows\twain_32\CNQL25" end of line.

Check is the you path right.

Follow the instructions and reboot the computer to complete installation.

perhaps if you run all exe by in Admistrator mode it would edit this path ?

Thaks for http://www.youtube.com/watch?v=rYQKh8WkAKE

Process is largely correct, but to get our 5200f working on Windows 8, there's a few differences:

(1) 5200f drivers will not be "found", so you must manually install these.

(2) The most up-to-date- CanoScan Toolbox software will not work and will continually fail with the message "unable to select TWAIN source".

So this is the process I used:

Install the device driver first:

                Run "5200fvst6410160a_64en.exe" by double-clicking it OR

                Open the folder "5200fvst6410160a_64en" then double-click "SetupSG.exe"

Install the CanonScan Toolbox:

                Run "cstbwin4932en.exe" by double-clicking it.

                Follow the instructions and reboot the computer to complete installation.

When the CanoScan 5200f is connected and switched-on, it will complete calibration actions when

the computer reboots. This is a sign that the device driver has recognised the scanner.

Latest CanoScan Toolbox software V5.012 installs correctly but fails to bind to the older device

driver. This is why the previous software version is installed instead.

Fixed mine. There are no Windows 8 drivers available on Canon's website or Windows Update. Follow these steps to install a Canoscan 8400F scanner under Windows 8 x64 in desktop mode:

(1) Download and install the Windows 7 x64 drivers from the Canon website.

(2) Launch Device Manager, right-click and uninstall any (?) unknown device, then rescan for new hardware and the Windows 7 x64 drivers will automatically install.

(3) Download and install the Canoscan Toolbox software from the Canon website.

(4) Launch the Canoscan Toolbox software, select Settings, and click the Reset button to repair the non-working Copy/Scan/PDF/Email scanner buttons.

(5) Reboot.

Button functionality is restored after the reboot. Everything works as designed now. Good luck!

Azskygod: you are a Saint.  Thanks for your excellent advice!