there might be a possibility that your firewall is hindering itunes from accessing your internet try this one if every time you open the itunes if you want to allow this program to make changes
try this article http://support.apple.com/kb/HT2553
I had the same problem, tried lots of things but this easy solution worked.
Go to the file location where Itunes is located (I had a shortcut on my desktop and just right clicked, went to properties and hit "open file location")
Once at Itunes file I right clicked again and hit "create shortcut"
It put a new shortcut on my desktop, I deleted the old one and the problem stopped.
Hope this helps