1. Ping the site or sites that are not loading. Simply go to Start/Run and type 'cmd' to open a command prompt. This is a little black box with white letters. Type 'ping' followed by the website address. Just type the domain, such as 'ehow.com.' 'Http://' and 'www.' are not necessary. If you are able to ping the site, but unable to load it, the DNS server is likely the problem. If you are unable to ping the site, you may have latency issues that you should take up with your internet service provider.
2. Flush the DNS cache. Open another command prompt. Type 'ipconfig/flushdns' right where the cursor is and hit enter. Try visiting the site that is giving you problems and see if this corrected the problem.

3. Edit the host file. The host file in Windows is a repository of website IP addresses. Your browser will check there first when trying to connect to a site rather than sending out a message to the DNS server. Open the host file with wordpad at C:\windows\system32\drivers\etc\hosts. Enter the problem website url and corresponding IP address, then try the site again.
4. Check to see that your DNS settings are correct. Contact your administrator if you are on a workplace network or your ISP if you are having trouble at home. They will help you check your setup to insure the DNS server information is correct.
5. Try changing the DNS server to something like Open DNS. Go to your router setup page. Accessing the setup varies depending upon your router or modem manufacturer. Check the packaging or instructions for details. For example, access most Netgear routers by typing 'routerlogin.com' in the address bar of your browser while a simple 'home' gets you to the setup for 2Wire modems and routers. (See Reference 3 below.)
6. Find your DNS setting and change it from 'get DNS settings automatically from ISP' to manual. Enter the server information for your chosen DNS, and try accessing the trouble sites again. This could not only fix your problem, but make your surfing faster going forward.