In the command prompt, type in:

ipconfig /release
and
ipconfig /flushdns

That will release the IP address for the specified adapter and purges the DNS Resolver cache.

If that doesn’t work, go into the Device Manager and right click on the Network Adapter and click Uninstall. Then shutdown your computer and physically remove the Adapter. Start your computer back up and log into windows. Check the Device Manager and make sure the Adapter has gone. If it has, shutdown the computer again and put the Adapter back in and then startup your computer again and log in.

if you are on win xp sp2 / vista / win 7

netsh winsock reset

You should notice that the New Hardware wizard starts or something similar. This should re-install the Network Adapter to defaults.