I don't know about windows, but on linux if you run xev it shows you the keypress and keyrelease events. I have a different numpad, which has a slider switch. The "windows mode" toggles numlock for every keypress, the "mac mode" does not (which is what I ended up using). There should be some windows utility that shows keypress events, if not, boot some live linux from a CD or usb stick and try "xev". Reading your numpad's manual might have further info too, there might be a key combination to toggle this behavior, or a driver option.