Are you using remote desktop? I am having a hard time wrapping my head around a link between the two computers that is not possible, other than for the idea you suggest (norton examining everything on the LAN). Are you sure you have a switch and not a hub? Because the other desktop shouldn't be getting the packets from your laptop if you have a switch.
No, I'm using a switch, not ahib; you know, hubs are obsolotete because they send and receive everything from and to all the RJ45 ports; switches are supposed a little smarter.
And yes, I'm using remote desktop, but disabled the lin between both computers and strenghten windows firewall in one of them for security reasons; I hope this solves the fan problema as a side effect.