Quote: "I think a fresh install is in order"
The zip file problem, i.e. the security settings set up when the file is created, occurs on both machines and the desktop is a fresh install so I don't see how that would help.
Quote: "Do you have Windows Defender switched on (on the other machine)? It could be this or another anti-virus program resetting the security if you have the drive mapped on another computer."
Why would that affect only zip files? The problem, as noted yesterday, arises when the zip file is created. It only came to light when I tried to copy the file to the other machine. Why would Windows treat zip files differently when they are created?
I've just checked several old zip files and none of them have this security/permissions issue. It seems to be a recent issue affecting both machines. Is there a Windows setting somewhere which enables you to choose the security setting for zip files?
Incidentally, when I create a zip file using the select/right click/send to sequence the new zip file entry in explorer has a small padlock symbol showing on it. That symbol disappears when I change the settings to my standard security settings. That's a recent occurrence.