function getXmlHttpRequest(){ 
         var xmlHttpRequest = null; 
         if ((typeof XMLHttpRequest) != 'undefined')  { 
               //非ie浏览器 
              xmlHttpRequest = new XMLHttpRequest(); 
         }else { 
               //ie浏览器 
             xmlHttpRequest = new ActiveXObject('Microsoft.XMLHttp'); 
         } 
         return xmlHttpRequest; 
} 
或者
function createXmlHttpRequest(){
        var xmlHttpRequest = null; 
        if(window.ActiveXObject){
            xmlHttpRequest = new AvtiveXObject("Microsoft.XMLHTTP");
        }else if(window.XMLHttpRequest){
            xmlHttpRequest = new XMLHttpRequest();
        }
}