Try a simple thing go out by a simple 4 port switch, hook one RJ45 from each Network Card into the switch the LAN part your ADSL, Cable hooks to that line. Look at the properties of your connect you can always use winipconfig /all after using run then CMD or winipconfig /? to find all the switches. If your ISP assigned an ID to your computer (Computer Name blahbalh-a) name the other the same name but change the last letter or number to the next higest A for computer 1 B for computer 2 or 1 for one 2 for 2.
If it is dial up use the Internet Connetion Sharing Wizard (like Bills Gate$ is Cool) It will ask you in their sucking M$ way which machine will access or act as the gateway then ask you to insert disks into A: to make the client for the second computer. More or less that is as simple as I will put it it is a no brainer I forgot the dial up back in the ISDN days