Is the Ethernet port onboard? If not turn off, remove it, boot back up, turn off, put it back in but use a different pci slot. Look carefully in Device Manager, instead of looking under "Network Adapters", look in "Other Devices" you may find it with a weird name or named "Unknown Device" and a yellow icon next to it if the drivers are messed up. Finally, if the green light is flashing, data is being send/received and the connection is active, your problem almost certainly is a software issue rather than a hardware one.
EDIT: Sorry forgot to mention, there is a chance that the port may be disabled in your BIOS, id definatly make this my next port of call if you havnt already tried.....Good luck