I experienced the same problems when trying to install on a Win7 x64 laptop an MP780 connected to a WXP x32 PC, but the suggestions in this thread did not word for me.
The issue appears to be that the Win 7 x64 driver package on Canon's U.S. web site only contains the scanner driver - the printer driver has been omitted! (Incidentally, Canon's U.K. web site only has Win 7 x32 drivers - described as the Win 7 driver - no x64 drivers are present.)
I did succeed in installing the printer locally, making use of the downloaded Win7 x64 drivers, with the laptop connected to the printer by USB. The scanner driver installed without any problem, and Windows Update intervened during the installation process, and automatically downloaded and installed the printer driver.
I then tried to install the network printer driver on the laptop (with the printer now connected to the network PC). The Add Printer dialog found the printer, but could not install it ("No driver found"). So then I followed the Add Local Printer workaround, as detailed elsewhere in this thread. I tried to create a local port, giving it the printer's network address as the printer name. The name was accepted, but when I was asked to select the appropriate driver, the MP780 printer driver was not listed.
The solution to this problem was to download from the Canon site the WXP x64 (yes, WXP) drivers. The extracted files included an MP780pr.inf. I tried the network installation again, and this time when Win 7 was unable to find the driver, I browsed to the extracted MP780pr.inf file. Win 7 accepted it, and installed the printer. It is now functioning perfectly.