Prueba a ver asi
Código PHP:
<?php
require('conexion.php');
$query = "SELECT * FROM usuarios";
$rs = mysql_query($query, $conexion);
$row = mysql_fetch_array($rs);
if($_POST['nick'] == $row["nick"] && $_POST['password'] == $row["password"])
{
session_start();
$_SESSION['nick'] = $nick;
$_SESSION['password'] = $password;
header("Location: index.php");
}
else
{
//crear y destruir session
session_start();
// vaciarla
$_SESSION = array();
// destruirla
session_destroy();
////////////////////////
echo "Login Incorrecto";
}
?>