Plug the mouse in while the pc is off. Then boot, and get to the login screen. Does the mouse work? If not, use the keyboard to log in, then open a terminal and su to root. Then type mousedrake and select each type of mouse in turn. After each selection, log out, then Alt-E to restart the x-server. If the mouse doen't work, repeat until it does work, or if it never works, try somebody else's suggestions