Have you done the usual connectivity checks: Can you ping the loopback address? Can you ping your own IP? Can you ping the router? If you perform a tracert, is it going to the right place? Have you swapped the ports and cables to check for physical damage? Have you updated the drivers? Have you swapped the cards around? If you have personal firewalls on the machines, you should add your network to the trusted network option of the software, rather than manually open all ports. Have you checked the machine for malware? When you do run ipconfig, do you get unusual characters appearing (wingdings looking things)?