Can't connect to Internet

I have a Dimension 3100C running Windows XP. Over 2 weeks ago I lost connection to the internet. I am connected to BT Broadband via their modem and an ethernet cable. I and a colleague have done the following:- 1. Checked the connection with BT - BT said there was no problem with the connection 2. Connected another PC to the modem and ethernet cable - connects to internet with no problems - so there are no problems with the modem or cable or internet. 3. Took my PC to a friends house and tried to connect to their wired internet via the Ethernet connection(Blueyonder) - couldn't get access. 4. Tried connecting to my friends wireless internet - connected to internet with no problem - so there are no firewalls or strange setup stopping internet access to PC. 5. I ran the hardware diagnostics on the Network card on my computer - computer states that it is working ok 6. Contacted Dell to say that my network Card wasn't working and they said as the driver is not reporting a problem the network card is ok, it must be a problem either with BT which we had ruled out or with the PC setup. 7. Tried connecting to BT via a USB connection instead of Ethernet - worked, I could access the internet. 8. Removed the hard drive and put in a new one and configured this to run with a wired network - still could not connect via Ethernet. Myself and my colleague are 99% sure its the Network card. I have spent over the last 2 weeks more than 10 hours on the phone to BT and Dell trying to fix the problem. Any suggestions of what I do next or what the problem could be?