CGI error when loading iFRAME

Hi, people #The problem: There is an error message that stops proper loading of one iFRAME, I couldn't understand what is the problem. Complete error message issued: >>CGI Error The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are:<< - In one of two FRAMEs of "main.PHP" , "zero.PHP" page is loaded. - "zero.PHP" has 3 iFRAMEs. - Both the first and the second iframe has its own self refering PHP file (if one of the PHP files name is "first.PHP" its form has: action="first.PHP"). - One of the PHP files, e.g.: "first.PHP" loads the "third.PHP" file in the third iFRAME when a condition was met, using expression like: echo '<script language="JavaScript">top.frames[1].frames[1].location="third.PHP"; </script>'; - Sometimes loading of the "third.PHP" issues mentioned CGI error. ##What is the problem, why the error? ###Additional: PHP Version 4.3.3 is running on Windows 2000 server