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After much trial and error, another user with the same issue as me (IJ Network Scanner Selector EX not being installed) found that booting into a clean state and/or turning off all shields would not allow a proper install. The only way to install the IJ Network Scanner Selector EX is to completely uninstall all the Canon software and drivers, fully uninstall Avast (use the Avast uninstaller from their website, not the Add/Remove Programs from within Windows) and then install the Canon software/drivers, check that you can scan and then reinstall Avast.