Use the "MG6400 series MP Drivers Ver. 1.02 (Windows 8.1/8.1 x64/8/8 x64/7/7 x64/Vista/Vista64/XP)", it should work (remove the spaces below in the URL):
http:// support-nz.canon.co.nz / contents/NZ/EN/0100540501.html
When you visit the page, scroll down to the bottom, select the checkbox and click download.
And they should take it back though. Should have at least a 14 day return policy depending on the store and I think required by law. Idk, you can always check.
Good luck.