i suggest clearing the network stack and then rebooting.
try right-clicking network neighborhood, delete everything under the "configuration" tab, then click ok. windows will ask you to reboot, go ahead and reboot and upon reboot it should find your NIC and reload drivers, as well as bind tcp/ip to your nic and whatnot. you can check it out by right-clicking network neighborhood and clicking properties. the settings you will need to verify are usually called "TCP/IP => (your network adapter)". most likely if you are connected directly to a cable/dsl modem you will want your ip address in the tcp/ip properties set to "obtain an IP address automatically". you might need to click the dns configuration tab to have that one set as automatic as well. click ok and after reboot open a command prompt window and see if you can ping yahoo.com.