try to reinstal the mouse drivers. go to start, control panel, system, hardware, device manager find the mise and other devices. click on plus mark. than right click on ps/2 compactible mouse and uninstal it. than you have to turn the pc off and on. the drivers will install again. maybe it will help to solve the prob.
the think is there could be another software installed on you pc that can cause the prob. if the uninstall wont work it may well be some other software conflicting with the mouse drivers.