Dual Network Cards on NT

Can anyone help me??

I have just had Cable Internet installed on my NT4.0 Server(SP6a) computer. It works just fine with only one network card installed, but when i install a second network card to access my LAN, it all stops working!!
The computer seems to get confused as to which network card it should use to access the internet – whichever card i set up to use cable, it will try to use the opposite one!! Does anyone know how to fix this?

Any advice would be greatly appreciated!

Cheers