Guess I got into the whole cut'n'paste game a bit too late. Or something.
But no, it was a software problem, as I first noticed it when I couldn't right-drag text to do an auto copy&paste in TeraTerm. From there I tried notepad, or any other app, and they all steadfastly refused to spare me a few bytes for copying and pasting. I could understand if perhaps I'd been running lots of things and Windows was slowly tying itself in knots, but the latest occurrence happened only about 5 minutes after booting up.