"the salesman suggested a Belkin wireless desktop PCI network card to fit to one of the desktops to act as the connection point for the laptop to the network. I've set this up on the P3, "

Sadly, I'm unfamiliar with this setup. I'd call your saleperson and ask how to make it work.

Usually I just install a wifi router which the DSL/cable connects to and shares the internet to the rest of the machines.

best of luck,