Yes, thank you very much for the rapid reply.
It does seem that the Preferences window is opening, but is not visible. That's exactly what I meant by saying that Esc frees this up.
I have no idea what I did to mess this up, and was hoping that it was something simple like a reinstall of some part of Windows or Calibre which the Preferences Window, Individual Title Editor and Tags Window have in common, since those are the three things acting up in apparently the same manner.
I reinstalled the Microsoft .NET Frameworks and the updates to them for 1.1, 3.5 and Client 4, since at least one of those was giving an error message, but to no avail.
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