1、客户端：
function callXMLHttp()
    {
    var strurl;
    var result;
       
    strurl = XMLHttpServer.aspx?strID=100;
   
    var objHttp = new ActiveXObject(Microsoft.XMLHTTP);
   
    objHttp.open(get, strurl, false);
    objHttp.send();
   
    result = objHttp.ResponseText;
    }

2、服务器端：
private void Page_Load(object sender, System.EventArgs e)
{
if (Request.QueryString.Keys.Count > 0 && Request.QueryString[strID].Length > 0)
            {
                strID = Request.QueryString[strID].ToString();
            }

            Response.Write(要返回的值);
}

还有一种写法：
function Test(objSrc,objDes)
    {
        try {
            xmlHttp = new ActiveXObject(Microsoft.XMLHTTP);
        } catch(e) {
            xmlHttp = new XMLHttpRequest();
        }

        xmlHttp.onreadystatechange = function() {
            if (xmlHttp.readyState == 4) {
                if (xmlHttp.status == 200) {
                    var data = xmlHttp.responseText;
                    document.getElementById(objDes).value = data;
                }
            }
        }

        xmlHttp.open(GET,CodeNameServer.aspx?testid= + objSrc.value);
        xmlHttp.send(null);
    }