lol, sounds exactly the same as my network.
in the device manager, under "network adapters" it should have, on the computer with the modem, the modem and ethernet card/onboard ethernet, and on the second computer it should just have the ethernet adapter. right click each one and uninstall. then at the top, click action > scan for new hardware. itll pick up the network adapters using either the windows drivers, or the drivers installed previously. it should run the network wizard too. if it doesnt, run it manually, making sure the workgroup name is the same on each computer. then go into control panel > network connections, there should be 2, one for LAN the other for broadband. they'll both say connected and one should say shared, the shared one being the broadband. if it isnt shared, right click it > properties > advanced > allow other users....... and of course, if it hasnt run automatically, run the same on the second computer, in the network connections folder, there should be 2 also, a LAN one and a gateway i think is the other one. make sure to follow the network set up correctly ie which computer is host etc. if it doesnt work post back. BTW your not with NTL are ya?