Solutions to errors » Why do I frequently get DNS error message?

I have a problem with regards to the Internet. I frequently get an DNS error message or simply “The page cannot be displayed” for IE8 and “Server Not Found” for Mozilla Firefox. I was very annoyed because I have a good internet signal. Two of the popular websites that can’t be opened due to this error are Microsoft website, Yahoo!, also even yahoo messenger. I can’t even use my windows update because it’s having an error. My desktop widgets that uses internet connection flashes on and off its content. And the most annoying, I downloaded Office 2010 beta (around 650Mb) at the Microsoft website but when it’s already 99%, then it says fatal error and the file downloaded disappeared.

I can still access these websites (that frequently says an error) for sometime that’s why I was able to download office 2010 beta but with several trials.

Can you help me deal with this? Can this be fixed or dependent on the service provider?
Thanks.