var XmlHttp=new ActiveXObject("Microsoft.XMLHTTP");

function sendAJAX()
{
    XmlHttp.Open("POST","Online.aspx",true)
    XmlHttp.Send(null);
    XmlHttp.OnReadyStateChange=Online;
}

function Online()
{
    if(XmlHttp.readystate==4 || XmlHttp.readystate=="complete")
    {
       document.getElementById("online").innerHTML=XmlHttp.responseText;
    }
}

setInterval("sendAJAX()",1000);