Here is the reply I got from Canon when I told them about this particular probem:
Thank you for your inquiry. We value you as a Canon customer and
appreciate the opportunity to assist you with your PIXMA MP780.
We have posted the latest Windows 7 drivers for the MP780 on our
website.
To get your PIXMA MP780 operating on your Windows 7 Computer, please
follow these steps:
1. Visit Canon's website at: http://www.canontechsupport.com
2. Just below 'Please choose your product from the menus below:',
select your model in the following manner:
2-1. Click the down arrow on the top drop-down box and select
Multifunction.
2-2. Click the down arrow on the middle drop-down box and select
Inkjet.
2-3. Click the down arrow on the bottom drop-down box and select
PIXMA MP780 and click 'GO'.
3. On the Support page for the your unit, click the 'Drivers &
Downloads' tab.
4. Choose [Windows 7 (x64)] from the Operating System drop down box.
5. Click on the MP780 MP Driver Ver. 6.20, [mp780vst64620en.exe].
6. Read the License Agreement on the next page, and if you agree,
click on 'I Agree - Begin Download' at the bottom.
7. A File Download box should appear. When it does, select Save.
Note: This feature may not be available in all browsers.
8. When the Save As box comes up, choose Desktop and click Save.
9. Once the download is complete, close all windows.
10. Double click [mp780vst64620en.exe] to begin installation of the
drivers.
11. Follow the prompts to complete the setup.
Please let us know if we can be of any further assistance with your
PIXMA MP780.
Thank you for choosing Canon.
Sincerely,
Michael
Technical Support Representative