Hmm, I wonder if it might have anything with Accessibility Options, although it seems awfully specific for that. And the Keyboard layout is US? I'm grasping at straws, but I know some keys behave exactly like your space bar in certain keyboard layouts (although it's usually with characters like ' " ^ etc, not space). I did find some WordPress related posts with similar issues (link), but nothing particularly promising in them. For some people the behaviour manifested itself after a certain (invisible) tag, for example after pasting (formatted) text and the like, but if it's like that whenever you start typing a reply, then it's not that. Did you change something in your sig and could it technically be related at all?!?!?! Is it in both editors (quick reply and the advanced/more options one)?