Bizarre keyboard/keyboard viewer behaviour

[JadeNB]

I have an old 15" Powerbook G4 (so PPC, obviously), running OS X.4.11.

Earlier today, in the middle of a Terminal session, my keyboard started behaving very bizarrely. When I pressed keys on the right-hand side of the keyboard, they would behave as if they were random other keys. (For example, when I pressed ':', it would behave like '-' --very hard for a vim user!) Unfortunately, I didn't make a chart of which keys corresponded to what. When I pressed keys on the left-hand side of the keyboard, nothing would happen.

I suspected that I had somehow selected some sort of weird keyboard binding, so I opened keyboard viewer. It showed the expected layout. When I pressed the ':' key, the ':' lighted up in the viewer. When I pressed a left-hand key (like 'q'), nothing happened at all. I was baffled, and couldn't really enter any commands (no key registered an alphabetic keypress!), so I rebooted in the hopes that it would go away.

It seems to have done, but now, whenever I am logged in to the account with which the original problem occurred and any authentication screen appears --for example, to log in to another account, or enter the administrator password --the keyboard viewer appears again. The little input-menu icon in the menu bar doesn't know this has happened; it still has the selection "Show keyboard viewer" available. If I click on that, nothing happens; if I then choose "Hide keyboard viewer" --or if I just close the viewer normally-- it goes away as expected, until the next time I get an authentication message.

The problem does not seem to occur with other user accounts on the same computer.

Any idea what could be causing this? A very old Toshiba I had once had a problem with the keyboard misbehaving erratically that turned out to be, literally, the battery overheating and melting the rest of the computer. I'm terrified of something like that happening again, and want to know whether I should anticipate an upcoming heat death. The computer (the Powerbook, not the Toshiba!) does seem to have been running a bit hot lately; but, according to Temperature Monitor, all temperatures are within acceptable limits.

I'm also posting this to MacOSXHints, so my apologies if you see this multiple times.