This is typically due to the fact that your computer is not connected to a network, _and_ you have not defined "localhost" in a "hosts" file.

If you are using Windows, find or create a "hosts" file in the appropriate place (do a search on Google for "Windows hosts file" if you've never heard of such a file, just for background info), and add the following line:

  localhost   127.0.0.1

Editor: On NT4, this file can be found here:

  C:\WinNT\System32\Drivers\etc\hosts