Help with network switch and ntl connection

I have an ntl cable modem (broadband at 600). I bought a network kit
consisting of a 5 port switch, two fast ethernet cables and two nic
cards.

I connected as per instruction this pc, using its onboard nic, to the
new switch, and the switch to my cable modem.

Everything worked fine as far as broadband connection.

Then I installed one of the new nic cards in a second pc, installed
its drivers and connected it to the new switch. Again, everything
worked fine for broadband, though I couldn't 'see' the other computer
from either machine in network neighbourhood.

I checked network neighbourhood and discovered I hadn't set a common
workgroup - so I did, in both, and rebooted both.

Now the first machine still connects to broadband fine but the second
machine doesn't; and neither machine sees the other.

What on earth did I do?? Help muich appreciated...