title: Could not establish trust relationship for the SSL/TLS secure channel
body: <p>I have developed an application which is looking at a 3rd party HTTPS endpoint. 
  On my personal PC (Windows 7) and the customers PC's (Windows XP) everything is working fine. </p>
  
  <p>On another test PC (Windows XP SP3-We are working with some legacy stuff) I am getting the following error.    </p>
  
  <blockquote>
    <p>The underlying connection was closed: Could not establish trust
    relationship for the SSL/TLS secure channel.</p>
  </blockquote>
  
  <p>This application does not use any client side certificates. There is no code which deals with certificates and I have not manually installed any certificates on any of the PC's.</p>
  
  <p>When I browse to the endpoint on my Windows 7 machine it comes up with a login screen as expected.
  When I browse to the endpoint on my WindowsXP machine it comes up with the message: </p>
  
  <pre><code>There is a problem with this website's security certificate. 
  Click here to close this webpage 
  Continue to this website (not recommended).
  </code></pre>
  
  <p>After clicking <code>Continue to this website (not recommended).</code> the login screen loads as expected but has a red address bar with <code>Certificate Error</code> displayed.</p>
  
  <p>All the solutions I have found talk about installing certificates or changing my code to accept any certificates.</p>
  
  <p>So why is this single test PC failing when the others work without any changes?</p>