Internet connection

[Alan Duval]

I have moved my home computer, which has Ubuntu 8.04lts installed, to an
office which has a router connected to the reception desk and office.
I find that I am unable to access the internet although the wiring is OK
as plugging in a laptop running Win XP to the same socket establishes a
connection.

ifconfig shows the same ip address as at my home site (192.168.1.100),
whereas ipconfig in the WIN XP laptop shows a different ip address
(192.168.2.5).

How can I change the ip address in Ubuntu 8.04 to the correct one?
Also, why doesn't dhcp automatically find the correct ip address?

Both sites have broadband connections.

Regards,

Alan Duval