Unable to open TWAIN source

The annoying Canon scanner error "Unable to open TWAIN source" is usally caused simply by the PATH environment variable not containing the appropriate value such as c:\windows\twain_32\SOME-SCANNER-DEVICE_NAME

I would create a new env var named scanner and give it that string value. You can then add %scanner%; to your path variable and be able to change scanners withot messing with the path variable.

If you need the fix take immediate effect without a reboot, open a cmd session on the directory of your scanner and lauch its EXE file from there.

Under Windows XP, a left-click for the PROPERTIES of My Computer will give you access to env var's via the Advanced tab.