Problemas de Session y mySql

[qbanos.com]

ahí va el código...

fichero incluido a principio de cada página
session.php


session_start();

if (!isset($_SESSION["sNick"])) {
session_register('sNick');
$_SESSION["sNick"] = "";
}

$time = time();
$limit = $time - 1440;

$idSession = session_id();
$myNick = $_SESSION["sNick"];

$query = "DELETE FROM sessions WHERE lastTime < $limit";
mysql_query($query);

$query = "SELECT * FROM sessions WHERE idSession='$idSession'";
$rs = mysql_query($query);

if (mysql_num_rows($rs) == 0) {
mysql_free_result($rs);
$query = "INSERT INTO sessions (idSession, userNick, lastTime) VALUES ('$idSession', '$myNick', $time)";
mysql_query($query);

}
else {
mysql_free_result($rs);
$query = "UPDATE sessions SET lastTime=$time, userNick='$myNick' WHERE sessions.idSession='$idSession'";
mysql_query($query);
}