phpexcel download shows virus content warning

Question

I have the following code for download the contents in excel using phpexcel. When I download the file,its showing a virus warning alert.If I tried to click the accept the risk button ,it will download the contents. I have removed the data from excel in order to check if its because of data .But still there is no change. I have added many headers in the script ,still no change.Please help me.

My php code is as follows:

 session_start();
 require '../config/config.php';
 $db = new Database();
 $conn = $db->connect();
 require_once '../../PHPExcel/Classes/PHPExcel.php'; 
 $objPHPExcel = new PHPExcel();
 $objPHPExcel->setActiveSheetIndex(0);
 header('Content-Type: application/vnd.ms-excel; charset=utf-8');
 header('Content-Disposition: attachment; filename="excelreport.xls"');   
 header('Cache-Control: max-age=0');  
 $objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel5');
 //ob_end_clean();
 $objWriter->save('php://output'); 
 ob_clean();