It might need drivers for it.
You can check by Right clicking My Computer, Properties, then select the Hardware Tab, and hit the Device Manager button and you might see something like this ->

Usually means it needs drivers. Google around the type/model of the mouse you have and find drivers for it.
Example: Googleing "Microsoft PS/2 Mouse Drivers"
This is just the mouse i have, but i've never needed drivers for it yet so far..