why mouse stops working when i connect the LAN cable?

hello friends,
my laptop??s touchpad stops working when i plug in the LAN cable, but starts working again the moment i take the LAN out.
what is strange is that an external mouse will keep working even if i plug in the LAN.
i am afraid someone has hacked my computer, or at least its a virus, though i have kaspersky 2011.
by the way i am using