Lately I got this error when I freshly installed Drupal 6. I googled and google how to fix the problem.
All you got to do is navigate through your windows file system if your using windows look for the folder c:/window/system32/drivers/etc/ and open “hosts” file , you must be an admin to be able to change this file.
If you are running Windows vista/7 , right click on notepad and choose “run as administrator” and look for that hosts file. Uncomment the bit where it says “127.0.0.1” removing the # number sign.
Reload the page and the error should now disappear.