Before start make a backup of your theme’s index.php. I used default theme’s files for this. Your template can have different files or codes.

Firstly we have to copy our real index.php twice. After that rename them with index-ie6.php and index-real.php. Clear index.php‘s code and place that code below;

<?php
if (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6') !== false) {
include(TEMPLATEPATH . '/index-ie6.php'); }
else { include(TEMPLATEPATH . '/index-real.php');
}
?>

After that we change index-ie6.php for IE6 users. I simply change have_posts function with a h3 :). Here is the index-ie6.php;

<?php
/**
 * @package WordPress
 * @subpackage Default_Theme
 */

get_header(); ?>

	<div id="content" class="narrowcolumn" role="main">
<div><h3>Sorry You're using Internet Explorer 6 </h3></div>

	</div>

<?php get_sidebar(); ?>

<?php get_footer(); ?>

Don’t change index-real.php’s codes. This is the real index.php of your theme.
That’s All. There is no more code :) If you want i take some screenshots for this. IE6, IE8 and Firefox.

Here is the code;

add_filter('template', 'serve_default_to_iesix');
add_filter('option_template', 'serve_default_to_iesix');
add_filter('option_stylesheet', 'serve_default_to_iesix');
function serve_default_to_iesix($theme) {
	if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6') !== false)
		$theme = 'default';

	return $theme;
}

Insert this code to your Theme’s functions.php. With this code we will make Browser detection and we tell wordpress ‘Change theme to default if our visitors are use IE6!‘.

You can change ‘default’ to classic or any theme name. You have to edit this lines for make changes.

$theme = 'default';