OK, you will have to bear with me as I try to explain the problem.
I have a BT Voyager 2100 Wireless ADSL router, with one PC connected wirelessly and one connected with a wire.
Strangely, the difficult one (The wireless connection) works fine. However, when I try to access the internet using the wired connection, I can't get a connection.
I am using Windows XP Pro SP2 for the wired connection, and I think it must be something I need to change on the router's config page.
PS - the bold words are key info! ;)