Could be a bad NIC.
Even if Windows shows it to be in good working order it could be bad. Do you have another NIC to swap out and try?
BTW, at first it sounded like malware infection that caused the problem, but if you have properly formatted and reinstalled Windows then that is unlikely, but still a possiblity. Sometimes malware can corrupt a part of Windows called Winsock 2, causing the internet connection to be lost. There are some utlities that can usually remedy this situation, but so will formatting and reinstalling Windows.
Check to see if the proper driver has been installed on the NIC and if Windows sees it as working properly.
Start > right click on My Computer > Manage > Device Manager
Then click on the NIC in the list (probably under Network Adaptors).
EDIT: Ok, after re-reading your post I see you swapped the nic already. But was that before reformatting etc? If so, and you now have a clean installation of Windows and still don't have a connection, try swapping it again. But follow the ideas on the rest of my post above as well. Let us know!
Edited by Albert Frankenstein, 25 November 2005 - 03:00 PM.