This was becoming a nuisance, explorer would open a new window every time I logged in (and if I didn’t close them they would continue to compound after each reboot).

A support article suggests a minuscule change in the registry is the cause:

Navigate the Registry to:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Win­dows NT\CurrentVersion\Winlogon

Find the value Userinit. This value should read: C:\WINDOWS\system32\userinit.e­xe, including the comma.

Why was the comma missing? Who knows, but at least Windows is somewhat under control now.