title: Crash when running a .NET 3.5 app on .NET 4
body: <p>Our application is built using WinForms on .NET 3.5</p>
  
  <p>This was tested specifically on Windows XP, although i am not sure whether it is OS related.</p>
  
  <p>Whenever i run it on a machine with .NET 4 only, it will crash, saying that it cannot find the .NET assemblies versioned 3.5.</p>
  
  <p>I am wondering why this does not happen automatically. For example, some 3rd party libraries install a policy of some sort, that when a newer version is installed, it will be used, even if your application was compiled against an older version.</p>
  
  <p><strong>UPDATE:</strong> The exact error message is:</p>
  
  <p><strong>"Unable to find a version of the runtime to run this application".</strong></p>
  
  <p>My questions are:</p>
  
  <ol>
  <li>Why is this not the same with the .NET framework?</li>
  <li>The solution is to add the  element in the configuration file? Any other solutions?</li>
  </ol>