title: Running .NET 3.5 apps on .NET 4 only systems
body: <p>This question has been asked before perhaps multiple times, but I can't get the typical solution to work for me. The <a href="http://msdn.microsoft.com/en-us/library/ff770241.aspx" rel="noreferrer">proposed solution</a> is to put this in the "app.config" file:</p>
  
  <pre><code>&lt;configuration&gt;
     &lt;startup&gt;
        &lt;supportedRuntime version="v4.0"/&gt;
     &lt;/startup&gt;
  &lt;/configuration&gt;
  </code></pre>
  
  <p>But with a simple "Hello World" program, I still get the error: "Unable to find a version of the runtime to run this application.".</p>
  
  <p>For some background, I'm using Windows XP (through Windows Virtual PC on Windows 7), and only installed the .NET 4.0 runtime on XP.</p>
  
  <p>After some research, I found this page:
  <a href="http://msdn.microsoft.com/en-us/library/ee941656.aspx#core" rel="noreferrer">http://msdn.microsoft.com/en-us/library/ee941656.aspx#core</a></p>
  
  <p>It mentioned "If your application configuration file is named application-name.config, rename it to application-name.exe.config. For example, rename MyApp.config to MyApp.exe.config.".</p>
  
  <p>I tried that, thinking it could solve the issue, but I still get "Unable to find a version of the runtime to run this application." when I try to run the .NET 3.5 (client profile or full) application on WinXP with .NET 4.0.</p>