"CreateProcess failed" when proc_open()

Hi! Could anyone help me. Here is my problem. I need to start new external process. I do it by the next code:

           $descriptorspec = array(
                0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
                1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
                2 => array("pipe", "w"),  // stderr is a file to write to
           );

           $hf = proc_open( $ex_path, $descriptorspec, $pipes );

$ex_path is a correct path to my executable. However, I get "CreateProcess failed" when it runs. I have IIS 6, running in its native process model. Also I have PHP 5.2.1. What's wrong? Please, help!