This has been the case unfortunatly for the past couple of months. I can't install new or update packages from Tools ==> Extensions and Updates I tried everything, yet I am unable to find the reason. I've tried :
- Access NuGet and download packages from "Package Manager Console" Install-Package = SUCCESS
 - Access repositories using Web browsers, Visual studion internal browser = SUCCESS
 - Same window in Visual Studio 2010 = SUCCESS
 
Visual Studio 2012 - Doesn't work

Visual Studio 2010 - Works!
