function check_username(username) {

	if (ajax) { 
		
		ajax.open('get', 'checkusername.php?username=' + encodeURIComponent(username));
		
		ajax.onreadystatechange = handle_check;
		
		ajax.send(null);

	} else { 
		document.getElementById('username_label').innerHTML = 'The availability of this username will be confirmed upon form submission.';
	}
	
} 

function handle_check() {
	if (ajax.readyState == 0) {

		document.getElementById('username_label').innerHTML = '';
		
	}
	if (ajax.readyState == 1) {

		document.getElementById('username_label').innerHTML = '<label>Checking</label> <img src="images/uploading.gif" align="absmiddle">';
		
	}

	if ( (ajax.readyState == 4) && (ajax.status == 200) ) {

		document.getElementById('username_label').innerHTML = ajax.responseText;
		
	}
	
}