How to use xmlHttp Request.send()?......

My code in java script is as follows:

str_xml+="<xml_to_be_submitted><request_xml><client_id>"+document.frmCallEntryAdd.cboCLIENT.options[document.frmCallEntryAdd.cboCLIENT.selectedIndex].value+"</client_id></request_xml></xml_to_be_submitted>";
  var obj_http=new ActiveXObject("Microsoft.XMLHTTP");
  var str_url="ClientModuleFetch.aspx";
  var str_http_method="post";
  obj_http.open(str_http_method,str_url,false);
  obj_http.SetRequestHeader("Content-Type","application/x-www-form-urlencoded");
  obj_http.send(str_xml);
  var str_reply=obj_http.ResponseText;


  var xmlResponse = str_reply;

  var objXmlDOM=new ActiveXObject("Microsoft.XMLDOM");

Can any body tell what i had gone wrong?................