The user-agent string that is sent in browser headers is stored in the registry of the server computer, as listed in the following table.

Version Registry Key
3.0 HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform\.NET CLR.3.0.04131.06
Note   The build number (04131.06 in the example) will change for the final release of .NET Framework 3.0.
2.0 HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform\.NET CLR 2.0.50727
1.1 HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\5.0\User Agent\Post Platform\.NET CLR 1.1.4322

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Test for NET Framework 3.0</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <script type="text/javascript" language="JavaScript">
    <!--
    var RequiredFXVersion = "3.0.04131.06";
    
    function window::onload()
    {
      var foundVer = CheckRequiredFXVersion(RequiredFXVersion);
      if (foundVer != null)
      {
        result.innerHTML = "This computer has the correct version of the .NET Framework: " + foundVer + "." + "<br/>"
          + "This computer's userAgent string is: " + navigator.userAgent + ".";
      } 
      else
      {
        result.innerHTML = "This computer does not have the correct 
version of the .NET Framework.<br/>"
          + "<a href='http://msdn.microsoft.com/windowsvista/default.aspx'>Click here</a> "
          + "to get .NET Framework 3.0 now.<br>"
          + "This computer's userAgent string is: " + navigator.userAgent + ".";
      }
    }
    
    //
    // Retrieve available versions from the user agent string
    // and check if any of them match the required version.
    //
    function CheckRequiredFXVersion(requiredVersion)
    {
      var userAgentString = navigator.userAgent.match(/\.NET CLR[ .][0-9.]+/g);
      if (userAgentString != null)
      {
        var i;
        for (i = 0; i < userAgentString.length; ++i)
        {
          var ver = userAgentString[i].slice(9);
          if (CheckVersion(requiredVersion, ver))
            return ver;
        }
      }
      return null;
    }

    //
    // Check if a specific version satisfies the version requirement.
    //
    function CheckVersion(requiredVersion, ver)
    {
      requiredVersion = requiredVersion.split(".");
      ver = ver.split(".");
      
      // Major versions must match exactly.
      if (requiredVersion[0] != ver[0])
        return false;
      
      // Minor/build numbers must be at least the required version.
      var i;
      for (i = 1; i < requiredVersion.length && i < ver.length; i++)
      {
        if (new Number(ver[i]) < new Number(requiredVersion[i]))
          return false;
      }
      return true;
    }
    
    -->
    </script>
  </head>
  <body>
    <div id="result" />
  </body>
</html>