I was recently called to a customer's house to setup a new Canon PIXMA MG5150 multi-function printer. The installation went well and soon enough, the machine produced a high quality test print. I was even able to share the printer with a Windows 7 netbook with no real trouble (there is no WiFi on this model), although it seemed to take a while to download and install the correct driver.
What didn't go so well was testing the scanning function. Whilst the machine would copy with no problems, and would scan through the Windows 'Fax and Scan' program, the 'MP Navigator Ex' software that came on the installation CD simply wouldn't load. No error messages appeared but nothing would come up on the screen when the icon was clicked. In addition, the 'scan' button on the device had no effect at all.
I tried the usual steps of uninstalling and reinstalling the software and also downloading and installing the latest version of both the driver and the 'MP Navigator Ex' software from the Canon website but this made no difference.
After a long time, I eventually discovered the fault: There was a conflict between the software for the customer's previous scanner software (a Dell multifunction) and the new Canon software.
Although I thought I had removed all the software for the device the Canon was replacing, there was a remnant left over that was stopping 'MP Navigator Ex' from starting correctly.
The problem was located in the folder 'C:\Windows\System32\twain32' Viewing of hidden / system files needs to be activated for this folder to show in 'My Computer' or 'Windows Explorer'.
It appears that this folder contains a sub-folder for every scanner device installed on the system. In this case, there were two folders: One for the Canon PIXMA device I had just installed and one for the old Dell. I tried to delete the Dell folder but got an error message as a file within the folder was in use.
I discovered through 'Task Manager' that each time I had double clicked on the 'MP Navigator Ex' icon, a new instance of the program executable had been loaded. After I had used 'End Task' to close each instance of 'mpnex40.exe', I was finally able to delete the 'Dell' folder.
As soon as I did this, the program opened with no trouble at all and the scanner functions worked normally, including the 'scan' button on the machine itself.