Xbox Network Connection Issue - Please Help

[#1]

I'm trying to connect my son's old style Xbox to allow him to join Xbox live, but I can't even get it to make a network connection.
I've got a Thomson TG585 v8 router, and I'm trying to use Homeplugs to make a wired connection.
My other son's Xbox is set up exactly the same way without problems but it seemed to transfer straight across when I moved from Orange a couple of months ago.
I am getting an error that the Xbox can't find an IP address to connect to the network. I've tried to manually set it to the same IP address etc as the Xbox that is working, but then get told I can't use a static IP address!
I've tried logging in to the Router through the internet, but I can't find anywhere obvious to change anything.
All suggestions appreciated.

[#2]

Hi there,
Try setting the IP static, but make sure it's different from any that are already issued to machines on the network (any machine, not just an xbox). Each item needs a different IP in order to work correctly, you can't have two devices on the same IP.
Hope that helps,

[#3]

Can you test the homeplugs with a different device (laptop/desktop) to make sure they are working as they should.  Have seen them fail sometimes due to house wiring.