I am trying to install Ubuntu 12.04 but when I run installer (or LiveCD) it writes that there is no connection to Internet (I don`t continue installation because ubuntu is the only OS on my pc and I need Internet access). Previous versions (10.04 and 11.10) work fine. My network adapter is ASUS NX1001. NOTE:I am loading from CD and run the installer.
I had this problem yesterday when attempting to upgrade, for me it seems that the connection to the Ubuntu servers was too to slow for the installer to connect properly. Either wait a few days until the hype calms down or try waiting till later on at night (when I upgraded mine) when the servers are less busy.
Another suggestion would be to try and find the best server for you area by having a look at this question. I found the second answer best as you can let the computer automatically choose the best server for your connection.
Alternatively you could download the .iso from the Ubuntu website, create a live CD or USB and upgrade that way.
Hope this answers your question.