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The setting in question is in the 'File Explorer Options' section of the Control Panel (the actual Control Panel, not the new Windows 10 Settings app). You can get to it quickly by opening the Start menu and typing in 'File Explorer Options', then hitting enter.
Under the 'View' tab, there will be an option in the list with a name like 'Use check boxes to select items'. Uncheck this then hit the Apply button, and it should fix things.
Pretty simple. Thanks, worked fine! I didn't set it explicitly, though. – Jack – 2017-07-21T18:57:42.163
I'll accept your answer in 8 minutes... – Jack – 2017-07-21T18:57:57.643