var request = null;

try {
request = new XMLHttpRequest();
}
catch (trymicrosoft) {
try {
request = new ActiveXObject("Msxml2.XMLHTTP");
}
catch (othermicrosoft) {
try {
request = new ActiveXObject("Microsoft.XMLHTTP");
}
catch (failed) {
request = null;
}
}
}

if (request == null)
alert("Ошибка создания объекта запроса!");