I spent hours having this same problem. Canon support was no help. They had me jump through hoops and even after talking to several 'senior engineer', they could not figure it out. Of course at that point it became my "system" that was at fault (ha ha).
I managed to dig up the driver install log file and found that the install was incorrectly searching for the file "usbscan.sys" in the folder C:\Windows\Drivers and C:\Windows\Inf
Well, guess what, it's not in either location on Vista!
I don't know if this is the location you will find it, but on my computer it was located in C:\Windows\System32\DriverStore\FileRepository\sti.inf_727905e0
If you don't find it in that location, just search your C:\Windows\System32 folder for it.
Once you've found it, copy it to those 2 folders,
Run install again and everything should go smoothly with no more stupid errors (at least it did for me).
Hope this works for everyone.
And just FYI, Canon told me "You are not the only one having this problem". So I took great joy out of being able to call them back and TELL them the solution to tell everyone to try. Maybe I should send them a bill
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