Answering my own question!
After googling around the forum, I discovered that other Windows users of Arduino IDE have experienced error symptoms similar to mine, with the cause often being a missing or erroneous "Personal" registry key at
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
So I used regedit to examine this key and discovered that it indeed pointed to a network storage location that I can no longer access (my laptop is a former college machine that was set-up by school IT staff). I'm no Windows guru, but it was dead simple to edit the "Personal" key to point instead to my local "My Documents" folder, after which the Arduino IDE opens and runs perfectly.
(Hoping this may be of help to others.)