Ah Duncan, you've been too clever for your own good. You've manually plonked a user program into system space.
Bit of explanation:
Operating systems take considerable care to protect sensitive parts of the system from the user, and users from other users. For example, a user is never allowed to write to someone else's memory. There are quite a few different 'users' on most systems other than real people, for example there's one to handle printing. One of the true negatives of MS-Windows is that domestic versions allow ordinary users to set themselves up as administrators. One bad effect is that warning messages are suppressed and the user isn't warned he's doing anything unwise. Admin privileges make life very easy until you make a mistake. Then most people discover they don't have the admin training needed to fix admin mistakes. This is why Windows has restore points; with luck it's possible to undo the mistake, though a restore will also throw away all your good changes.
When you run a Windows Installer it does more than unpack files and copy them on to the hard drive. It also knows what permissions are required and sets them appropriately. By installing into the shared system folders, it can make the program available to all users on the computer. The installer likely does a lot of other clever stuff too, such as dependency checks, creating icons and setting the Registry etc.
It happens that the Arduino IDE can also be 'installed' by a single user copying it into one of his private folders . This 'install' is a simple copy. When done that way, the permissions don't need any special treatment. It just works. The down side is that the installation is private to the user who installed it, not shared. (This probably doesn't matter.)
Sounds as if Duncan's installation created a hybrid. The IDE was manually copied into system space without making the necessary changes to the permissions. The IDE works anyway, but there's an unexpected permissions problem when Duncan tries to write a file.
Fixing this manually is certainly possible but it might require a lot of detective work. Instead I'd delete the Arduino folder, and re-install the IDE using the Windows Installer Option. That should rebuild the IDE under ProgramFiles with the right permissions.
Once that's done the problem should disappear. Fingers crossed and don't forget to sacrifice a virgin first.
Dave
Edited By SillyOldDuffer on 29/09/2017 11:22:39
Edited By SillyOldDuffer on 29/09/2017 11:24:38