I am heading out for the weekend, taking my 10 year old to the observatory, so I wont respond till monday.
But before I go I wanted to reply
Check the event viewer and see if anything is logged about the network, any errors or anything.
Check the services and make sure the server and workstation services are running. Also make sure the firewall is off, on a server it is normally off, if you like it on lets turn it off for testing.
I have been on servers where the server firewall would stop a lot of things even when it was set to allow them through.
I know there are some other services that need to be running for the network to work like normal, lanman and others, I just cant think of all of them at this moment.
Also, just to cover all the bases, if you have another network card you can use just as a test, install one and see how it does. That would at least confirm if the onboard nic and the drivers are good or not. If it happens to a totally different nic and drivers then we can mark that off the list as a possible cause
Basically at this point it is a process of elimination
Shane