var http = false;

if(navigator.appName == "Microsoft Internet Explorer") {
	  http = new ActiveXObject("Microsoft.XMLHTTP");
} else {
	  http = new XMLHttpRequest();
}

function validate(user) {
	http.abort();
	http.open("GET", "ajax/ajax_validate.php?name=" + user, true);
	http.onreadystatechange=function() {
		if(http.readyState == 4) {
			document.getElementById('foo').innerHTML = http.responseText;
		}
	}	
	http.send(null);
}