Canon stopped supporting this scanner after XP. The available software and drivers are here: http://www.canon-europe.com/Support/Consumer_Products/products/scanners/Others/N640P_ex.aspx?type=download&page=1
Depending on the version of Win7 on your computer, you can try one of the following workarounds. To check your version, click Start > Control Panel > System and Security > System. If it says x64, you have a 64-bit version of Windows 7. Otherwise, it is the 32-bit version.
1) If you have the 32-bit Win7, you can try the compatibility mode. Download the driver or full software from the Canon site and right click on the installer. Then choose Properties and then the Compatibility tab. Turn on Run program as Compatible with the appropriate OS. Then right click again on the installer and choose Run as Administrator. Then check if the program itself is running in the compatibility mode by right clicking again and choosing Properties. This may not work.
2) The option that will work for all versions of Win7 is to use a computer (virtual, dual-boot, or true second machine) with an older version of Windows installed. If you have Win7 Pro or Ultimate (or upgrade to them), you can create a virtual XP computer for free. Download and install Virtual PC and XP mode from Microsoft. Install them in order. Then install the XP software from the Canon site in the virtual computer and it will work. (Other virtual computer programs are Parallels and VMWare.) All of the other options will require a licensed version of the older Windows OS. Please note that the virtual computer can't share files directly with the main computer. You'll need to save files to external or removable media (optical media or flash drive, for example) to share them. Also a printer cannot be installed on both the real and virtual computers at the same time. You will be prompted to uninstall one set of drivers if you try.
I hope this helps.
(the above suggestions assume you do not want to replace the scanner which would be faster.)