树青兄写的一个ajax类。
var http_request = false;
function send_request(method,url,formName,responseType,callback) {
 http_request = false;
 var content =""
 if(forName != null)content = getFormAsString(formName);
  
 if(window.XMLHttpRequest) {
  http_request = new XMLHttpRequest();
  if (http_request.overrideMimeType) {
   http_request.overrideMimeType("text/xml");
  }
 }
 else if (window.ActiveXObject) {
  try {
   http_request = new ActiveXObject("Msxml2.XMLHTTP");
  } catch (e) {
   try {
    http_request = new ActiveXObject("Microsoft.XMLHTTP");
   } catch (e) {}
  }
 }
 if (!http_request) {
  window.alert("不能创建XMLHttpRequest对象实例.");
  return false;
 }
 if(responseType.toLowerCase()=="text") {
  http_request.onreadystatechange = callback;
 }
 else if(responseType.toLowerCase()=="xml") {
  http_request.onreadystatechange = callback;
 }
 else {
  window.alert("响应类别参数错误。");
  return false;
 }
 if(method.toLowerCase()=="get") {
  http_request.open(method, url, true);
 }
 else if(method.toLowerCase()=="post") {
  http_request.open(method, url, true);
  http_request.setRequestHeader("Content-Type","application/x-www-form-urlencoded");  
 }
 else {
  window.alert("http请求类别参数错误。");
  return false;
 }  
 if(window.ActiveXObject) http_request.setRequestHeader("If-Modified-Since", "0");
 http_request.send(content);
}

function getFormAsString(formName){

returnString ="";

formElements=document.forms[formName].elements;

for(var i=formElements.length-1;i>=0; --i ){

returnString+="&"

+escape(formElements[i].name)+"="

+escape(formElements[i].value);

}

}