Can't Call The Servlet Using AJAX

I create some javascript to send some search request. However, on some machines i wont' be able to send request. What could be the reason?

function sendSearchRequest()
{
var srch_url = "/servlet/Search";
try {
request = new XMLHttpRequest();
} catch (trymicrosoft) {
try {
request = new ActiveXObject("Msxml2.XMLHTTP");
} catch (othermicrosoft) {
try {
request = new ActiveXObject("Microsoft.XMLHTTP");
} catch (failed) {
request = null;
}
}
}

request.open('GET', srch_url , true );
request.onreadystatechange = GetSearchResponse;
request.send(null);
}

function GetSearchResponse()
{}