"Connection for controluser as defined in your configuration failed" with phpMyAdmin in XAMPP

I have just installed XAMPP on my Windows XP machine, and I get an error saying:

Connection for controluser as defined in your configuration failed.

Before I installed XAMPP, I had a MySQL database installed and it had a password. I changed and put the password in config.inc.php for MySQL, and I got this error:

<?php
if (!empty($_SERVER['HTTPS']) && ('on' == $_SERVER['HTTPS'])) {
    $uri = 'https://';
} else {
    $uri = 'http://';
}
$uri .= $_SERVER['HTTP_HOST'];
header('Location: '.$uri.'/xampp/');
exit;
?>

When I try to access index.php, it seems like something is wrong with the XAMPP installation. What can I do to fix this problem?