You need a cable modem with an Ethernet connection and a device called a router.
You connect the cable modem with the in-port (WAN-port) of the router and the outgoing ports (LAN-ports) of the router with the Ethernet-connection of the PC's. The usual routers on the consumer market support 4 PC's wired. A wireless router suppports a much larger number of PC's that have a wireless Ethernet receiver (laptops come with it, generally, for desktops you need to buy them separately).
The additional advantage is that the PC's are connected in a home network, so it's possible to share printers and files, if you like. Although this might be difficult between a Mac and a Windows PC, as you have.
It depends on the connection speed of the cable and what the PC's do with it if you'll see a downfall in service. You can't look high-res streaming video (2 Mb/s) on 2 PC's simultaneously if you've got a 2 MB/s connection, of course. But just browsing and mailing and listening to 96 kB/s streaming audio will go very well with 2 PC's and a 2 Mb/s broadband.
You'll find a wired router for some $50 to $60 in any computershop. It's a very common setup. Every shop assistant worth the title will sell you the right equipment and the right cables or cards.
Hope this helps.
Can you split a cable connection to go into two computers that can both be running? If so, what with and would the service be slower or same?
And if you can't do that, is there another way to run two computers (side by side) at the same time. Seems to me there would be conflict in doing this but you never know. thanks
(Windows XP desktop and Mac note book)