The easiest way you can possibly to it would be to *purchase* something like Deerfield´s "Wingate" NAT/Proxy software and run it on your machine and hook up an AP (access point) directly to your machine and bind the network interfaces to one another.
Alternativley you can purchase routers with a serial connection for an external 56Kbs modem hookup, however check the box before buys as most use this kind of connection solely as a console port. I used to have one a couple of years ago when cable was very unreliable in the uk, my network would quite happily (but slowly) fall back to 56k.
hope this helps