I had been on line for several hours and all of a sudden the mouse would only move up and down in the exact middle of my screen. I have unplugged, replugged, uninstalled, reinstalled, etc. I didn't have another computer or another mouse to try but will borrow a mouse this afternoon to see if that is the issue. In the back of my mind - I may have held down keys and mouse buttons at the same time...could a multiple key stroke have caused this? It isn't erratic movement - just vertical.