A friend of mine had a similar problem and this fixed it (on a Mac):
At the top menu, click Go.
Select “Go To Folder” from the submenu.
Open the folder “etc” and locate the “hosts” file.
Copy it to your desktop > right click > show info > make sure you have the rights to read and edit it.
Open it in text editor.
Search “184.108.40.206 gs.apple.com” and “127.0.0.1 gs.apple.com” (somewhere at the bottom normally)
Add a “#” and a space before it and save the file.
Copy it back into the “etc” folder and replace the old “hosts” file.
Restart iTunes and it should be fixed!
I found this solution on the Apple discussion board, sadly I can't post a URL since I do not have enough posts yet.