Problems while using ubuntu 12.04 to connect to the network

[NightKnight]

Hello, everybody, I've got this problem for a few days, and it drives me crazy!!!

I used Acer 4741g, and I updated my pc from 11.10 to 12.04 three days ago, then I found that its' hard for me to surf the internet.

When i use Chromium, it will tell me the webpage is not available, because of dns lookup failed,
for the firefox, it tell me server not found.
Even when i used apt to update or download softwares, it tells me that "Something wicked happened resolving 'deb.opera.com:http' (-5 - No address associated with hostname)".

I tried to set the dns server address manualy, but still not work.
Of course, the network is ok, the 11.10 works well, and my friends' Windows works well too.

anybody can help me? appreciated for any suggestions...