Recently I discovered that the TWAIN scanner driver for my Canon MF4690 printer/scanner stopped working. The WIA driver continued to work. I do not use this scanner very often, but it was working as late as May 31, 2008. It is my belief that one of the Windows updates I installed since that date disabled the TWAIN driver. The symptom was that when I clicked any of the buttons in the MF Toolbox window nothing would happen. I finally determined that the cause of the failure was that MF Toolbox could not find the TWAIN drivers that were located in the c:\windows\twain_32\mf4600 folder. I was able to solve the problem by editing the system environment variable PATH to include that folder. To edit the system PATH, choose the System control panel, click on "Advanced system settings" on the left side, and click Continue in the User Account Control prompt window. You should now have the System Properties window. Click the Environment Variables button and select the "Path" environment variable in the System variables list on the bottom. Do not make the mistake of choosing the Path variable in the User variables list. Click The Edit button. This will bring up the Edit System variable window. Scroll all the way to the right side of the variable value box and add ";c:\windows\twain_32\mf4600" to the right hand end of the value. Do not include the quotes but do include the semicolon. If you have some other model scanner this might work for you too, but you might need to change the "mf4600" to whatever name your TWAIN drivers are installed in.

Thanks for that info. I saved it for future reference.