Mouse problem on Dell Inspiron

I formatted and re-installed Windows 2000 on a Dell Inspiron 5000e laptop and everything works great except for the following:
When the laptop is docked in the docking station the external mouse works fine, but when it is not docked, the same PS/2 mouse does not work when I connect it directly to the laptop’s PS/2 port.

It worked fine before a re-installed Windows?