function update_message( transport ) {

    var json = transport.responseText;

    var response;
    try {
      eval( 'response = ' + json );
    } catch (e) {};

    $('errorarea').innerHTML = response.message;
}

function check_loginname() {
    new Ajax.Request( 'pdesk.cgi', 
                     { method: 'post', 
                       parameters: { do: 'ajax_checklogin', login: $F('username') },
                       onSuccess: update_message
                     });
}