OK, I admit the probability of accidentally pressing a key is a lot higher for CapsLock than for NumLock... But still, have you *positively* ruled it out?
To facilitate finding out, have you set up "ToggleKeys" in your "Accessibility Options"? They make the computer go "BEEP!" when you toggle a lock on, and "BOOP!" when you toggle it off; they're invaluable.
(For all of us fumble-fingers, that is, not just for gimps. Fuck knows why they're buried under Gimp Options; they should be on by default on every Winders box.)