You will need to find the TCP address of your cable modem.
in explorer on the win machine type
to talk to your cable modem... xxx.yyy.zzz.ooo is the address of your cable modem.
This should give you the menu to control your modem. Part of the info you can discover includes the DNS server address (also nnn.nnn.nnn.nnn)
Use the netconfig command on the linux box to set the gateway address to the address of your cable modem. Also add the DNS server address you saw on the windows box talking to the modem.
ALTERNATELY... use netconfig on the linux box to select DHCP and then OK until you get the command prompt again (and restart your machine, booting linux..)