Strange problem.
May be a compatibility problem with your USB keyboard. The game would not recognize the key since it's not the same than the old 486 computer keyboard.
I remember with my old 486 that I had spilled something on my keyboard, and as a result several keys stopped functioning. I countered this problem by using ASCII codes. For example I still remember today that e was alt-130 or alt-135
If someone knew what is the ASCII code of num lock you could try it that way.
Otherwise I have no solution for you, sorry.
