I too would expect this to be a hardware problem. Have you tried another keyboard? Or just unplugging the keyboard and see if the scrolling stops? Whenever something like this happened to me I had a stuck key somewhere or some dirt or hair stuck in the optical sensor of the mouse. Alternatively, have a look and see if there are other input devices still active. I have also forgotten to switch off my mouse at times and if this then gets stuck between some stuff in my bag, I get strange things happening on my laptop.