it this is a laptop be sure to disable the touchpad. Occaisonally this happens to me and its when I've accidentally turned on the touchpad and my hand brushes the touchpad as I'm typing. On my Acer the fn-F7 key enables/disables the touchpad. Other than that I would suspect a stuck key.
oh, and run a virus check, Could there be some remote control software on the PC where somebody is driving you mad by messing with the mouse?