Since I must be aware of the current password in order to open Windows, why on earth does the system prevent me from cancelling the feature ?
It's a security feature. Otherwise anyone could cancel the need for a password and defeat the whole security concept.
A Google search lists quite a few fixes to change the login password - some appear to be more simple that others. I haven't tried any of them, but most of them seem to have worked for other users.
Finally - or perhaps rather as a first step - persist for a while with what you think the password might have been. Good luck - it appears to be do-able.