I know this is an old thread but I ran into this exact same problem, on Windows 7 Ent. 64 bit. It occurred on my laptop, after I used a paper towel to wipe off my keyboard. As I was at the login screen, I couldn't see what was being typed but the only key that would type anything was the backspace key. if this was some key combination I pressed accidentally, what kind of feature is that? Who would want their keyboard to behave this way. Anyway, I guess I pressed enough keys which somehow corrected the problem. Very weird.