iMac not connecting to internet

I have an iMac that is not connecting to the internet. It is able to connect to my Airport network, but the Airport utility shows that the network is not connected to the internet.

This is incorrect, as I have a Mac Air and other devises that connect perfectly, and airport utility shows that same network as connected to the internet.

I've also tried a wired connection via ethernet cable, with no luck. I've even tried a bluetooth connection to my iPhone, with no luck.

It was working fine until I tried to upgrade the software. I had not used the computer for a while as I moved from London to the New York and the computer was in storage. When I unpacked used the computer originally it was fine, then I tried to upgrade and it lost the ability to connect. The iMac is a mid-2010 version currently running OS X 10.8.3 (12D78).

As a note, I've compared info in System Preferences/Network and Airport Utility, such as DNS servers, with my Mac Air, and there does not appear to be any differences. Only difference is that in airport utility, if I click on internet which my Airport express is connected, it says "disconnected."

As a last note (promise), when I restart the computer, it is able to connect to the internet for about 10 seconds, then the interest goes dead again.

Thanks in advance for any help.