Alan, thanks for your reply.
Yes, I'm running SP1. Just had yet another email from Canon tech support.
Now they tell me that I'm the only one describing this problem and they've got a 64 bit Vista machine to install the same printer with no problems. So, it's a "computer specific" problem now!
I'm thinking their "support" is a joke. So, it's back to the trusty old USB.
ps. I appreciate your help re. splwow64.exe, I should have said SysWOW64 folder stuff.