Here’s a simple solution to those facing Fresh Magento Installation Backend login problem. It’s obviously an issue around Cookie settings. So let’s get to it.
First thing you got to do is look for a file named Varien.php which is located same way as the following.

[magento_url]\app\code\core\Mage\Core\Model\Session\Abstract\Varien.php

I managed to get it to work by commenting the whole block lines 77-104:

/* $cookieParams = array(
‘lifetime’ => $cookie->getLifetime(),
‘path’ => $cookie->getPath(),
‘domain’ => $cookie->getConfigDomain(),
‘secure’ => $cookie->isSecure(),
‘httponly’ => $cookie->getHttponly()
);

if (!$cookieParams[‘httponly’]) {
unset($cookieParams[‘httponly’]);
if (!$cookieParams[‘secure’]) {
unset($cookieParams[‘secure’]);
if (!$cookieParams[‘domain’]) {
unset($cookieParams[‘domain’]);
}
}
}

if (isset($cookieParams[‘domain’])) {
$cookieParams[‘domain’] = $cookie->getDomain();
}

call_user_func_array(‘session_set_cookie_params’, $cookieParams);

if (!empty($sessionName)) {
$this->setSessionName($sessionName);
} */