Install trouble on shared hosting using Microsoft IIS and Plesk

<?php
/**********************************************************************
    Copyright (C) FrontAccounting, LLC.
    Released under the terms of the GNU General Public License, GPL, 
    as published by the Free Software Foundation, either version 3 
    of the License, or (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  
    See the License here <http://www.gnu.org/licenses/gpl-3.0.html>.
***********************************************************************/
        error_reporting( E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_ERROR | E_WARNING | E_PARSE | E_USER_ERROR | E_USER_WARNING | E_RECOVERABLE_ERROR );
        ini_set('display_errors', 1);

    $path_to_root=$_SERVER["DOCUMENT_ROOT"];
        echo "index.php: path_to_root = ".$path_to_root."<br />";

        if(is_writable($path_to_root)){
                echo '&nbsp;&nbsp;It IS writable<br />';
        }else{
                echo '&nbsp;&nbsp;It is NOT writable<br />';
        }

    if (file_exists($path_to_root.'/install.html'))
        echo 'index.php: File '.$path_to_root.'/install.html exists<br />';
    if (!file_exists($path_to_root.'/config_db.php'))
        echo 'index.php: File '.$path_to_root.'/config_db.php does not exist<br />';
/*
     if (!file_exists($path_to_root.'/config_db.php'))
        header("Location:".$path_to_root."/install/index.php");

    $page_security = 'SA_OPEN';
    ini_set('xdebug.auto_trace',1);
    include_once("includes/session.inc");
    add_access_extensions();
    $app = &$_SESSION["App"];
    if (isset($_GET['application']))
        $app->selected_application = $_GET['application'];

    $app->display();
*/
?>

index.php: path_to_root = G:\PleskVhosts\rad129.net\pssafrontacc.rad129.net
  It IS writable
index.php: File G:\PleskVhosts\rad129.net\pssafrontacc.rad129.net/install.html exists
index.php: File G:\PleskVhosts\rad129.net\pssafrontacc.rad129.net/config_db.php does not exist

<?php
/**********************************************************************
    Copyright (C) FrontAccounting, LLC.
...    See the License here <http://www.gnu.org/licenses/gpl-3.0.html>.
***********************************************************************/
$page_security = 'SA_OPEN';
$path_to_root=$_SERVER["DOCUMENT_ROOT"];
echo "install/index.php: path_to_root=".$path_to_root."<br>";
die("Dies intentionally in install/index.php");

if (file_exists($path_to_root.'/config_db.php'))
    header("Location: $path_to_root/index.php");
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