hhmm....well you should just be able to configure the ethernet port that is connecting to the cable modem with the command:
ip address dhcp
bring the interface up with the command:
no shutdown
this will allow the router to get an ip address through dhcp. then you should configure a route that points out the correct interface, something like this:
ip route 0.0.0.0 0.0.0.0 ethernet0/0 (it may be a different ethernet port..)
Erik Rudnick, CCIE No. 9545
mailto:erik@kuriosity.com