Oh, already encountered that problem. I don't know if this works, try pressing CTRL. I suspect that the problem is the mouse itself, not the pc. Try checking ur mouse connection, try bending or re-inserting ur mouse usb cord. Or maybe, ur pc is the problem. Try rebooting, or maybe someone messed up the configuration of ur mouse. Go to CONTROL PANEL and find MOUSE SETTINGS, configure it yourself coz I don't know ur pc. If still not fixed, buy a new mouse, because maybe someone used it as a projectile and threw it to someone and just returned it there without u knowing...