a me questo funzia bene

codice:
<%@LANGUAGE = JScript%>
<%
    var url = "http://www.html.it";
    try
    {
        var xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
            xmlhttp.Open("GET", url, false);
            xmlhttp.Send();
        if (xmlhttp.Status != 12007)
        {
            Response.write("Dominio occupato");
        }
    }
    catch (e)
    {
        Response.Write("Dominio libero");
    }
%>