If you've found yourself hitting DNS errors or 404 pages in your browser but you know your net connection is working fine, it might be time to flush and restart Windows' DNS cache. The How-To Geek shows how to do this in three command line entries, although you might only need the first to see results. After launching a command prompt as an administrator and closing down Firefox (which has its own DNS cache), enter the following:
net stop dnscache
net start dnscache
The Geek also shows you how to restart the service in Windows XP or Vista's DNS service from the graphical services menu.