First, disable driver signing enforcement:
From the Charms bar, select Settings then Change PC Settings (bottom right of the screen).
Select General and scroll down to Advanced Startup and click restart now.
From the menu, select troubleshooting then Advanced Options then windows startup options.
Restart again and select number 7 Disable driver signature enforcement.
Your PC will boot up as normal.
Now Right Click the Windows 7 driver file that you downloaded from the Canon website and select troubleshoot compatibility.
Go through the procedure and tell it that the driver worked in Windows 7.
It should then install and work.
Once you reboot again the driver enforcement is reverted but the printer still works.
(via the comments on DP Review)
