This can be a number of things and unfortunately, without seeing the machine, it is hard to advise.
The first thing I would do is check for a driver update for your network card as I have seen problems like this in the past.
Next, I would check that the adapter is being correctly discovered as if it is internal, there could be some sort of hardware problem - Right click on My Computer to go to Computer Management and then expand device manager and make sure it is listed under Network Adapters AND there is no sort of yellow triangle - if there is a triangle, try to see what the error is.
If it does not show under Device Manager, take a look in the system event log as you should see a few warnings.
Lastly, if it is found but there is just some random error - try going to Control Panel > Network Adapters then right clicking on your device and choose Disable then re-enable it.
As an extra, sometimes DHCP can just mess up in some environments - it may be worth trying to assign a static address to your machine as it is possible that everything is working but DHCP is not assigning a correct address. Without seeing your network, I can't say what address to try.
