t sounds like you have an issue with the installed Canon scanner driver. You mentioned in the other post that when opening Print & Fax and selecting the MP560 under the Scanners heading, the Open Scanner button resulted in a failure. This would stop every other application from working so if the mentioned Software Update doesn't help, delete the current scanner entry in the Print & Fax > Scanners section, download the Scanner Driver for the MP640 from the Canon web site and install. Then add the Scanner again and test using the Open Scanner function.
Have you tried Image Capture?