This keeps happening to me from time to time. I'm doing one thing or other, typing, and in the middle of it I pause, thinking... And I just happened to be pressing Shift, Alt, Ctrl or some other control key at that moment, and after a few seconds (probably 5) computer starts beeping and it turned on some accessibility option. It's not hard to turn off, but damn it breaks my train of thought with that beeping.

So now I always turn those options off in Control Panel as soon as I install Windows. It also protects my computer from blind hackers.