Best possible solution - though not cheapest - is an external router with hub.
I started off with a 4 port ethernet hub and connecting cable - really cheap, and great for sharing files between mine and my lady wife's computers. Couldn't get the hang of WinXP's Internet connection sharing though.
I then bought a NETGEAR ADSL Firewall router DG834, £60 approx. from NovaTech. It has an ADSL 'modem' and a four port ethernet hub built in. Good instruction set, just minutes to set up.
If you haven't already got an ethernet (LAN) port on your machine you will also need an adapter plug-in card. They aren't expensive.
With this set up, the ADSL link is available independently to either machine, a godsend if either crashes - as mine just did - or is just switched off.
If you got an extra £20 or so, there is a 'wireless' version of this router. Don't know much about that set-up. I assume you wouldn't need to drill holes in your upstairs and downstairs walls like I did, but you will need wireless adaptor cards!