External Net Connection to Comcast Cable Modem

I have a new install of server2003. I hope later to use ISA2004 for my
firewall.

I use Comcast Cable as my ISP.

Using Network connections I have setup a connection using both "Obtain IP
and DNS automatically". No other entries under ADVANCED have been set.

When I ENABLE the connection I receive an error that the DHCP server
(@Comcast) did not provide a valid IP address.

If I do the same connection setup on an identical machine running XP pro SP2
I get a good connection!

Then if I use these IP, Gateway, and DNS numbers as FIXED in the server2003
machine I can get a connection untill Comcast renews the IP lease.

What is different between XP and 2003?

Comcast support will not discuss this problem.
--
Jim