I think you might find it cheaper and easier to use a hub and set your winBox as a router or use a proxy connection.
The Hardware you would need would be cat5e ethernet cable to go between the comps and the hub, ehternet cards for both boxes unless you already had them (many new comps come with them. They look like big phone jacks.), and a network hub.
Then the layout is:
tv-cable eth eth
ISP------------ WinBox (acting as router) ----- HUB ----- Linux
Linux and win can talk, ISP and win can still talk, and Linux can now talk to the ISP through the win.
This idea is basically the same as the previous poster's, except that I think that on a network of yours' size, you don't need a physical router; the winBox should be able to do it with software.