Permission Denied?

I'm trying to use the xmlHttpReq object to contact a URL on my server.
But I'm getting a JS "Permission denied" error at the indicated line.
What does it mean and how can I get around it?

function signUp(signUpElt) {
if (!ValidEmail(signUpElt.value)) {
alert("Please enter a valid email address.");
return;
} // if

var xmlHttpReq = false;
var self = this;
// Mozilla/Safari
if (window.XMLHttpRequest) {
self.xmlHttpReq = new XMLHttpRequest();
}
// IE
else if (window.ActiveXObject) {
self.xmlHttpReq = new ActiveXObject("Microsoft.XMLHTTP");
}
var strURL =
"http://mydomain.myhost.com/add_email_response.php?EmailP=" +
escape(signUpElt.value);
self.xmlHttpReq.open('GET', strURL, true); // throws
"Permission denied" JS error.