First Solution
Replace the http://localhost/magento/index.php/admin from the i.p. address of the localhost then it will look like as http://127.0.0.1/magento/index.php/admin. I made it work into whenever I changed my browser. Suppose I installed magento using google chrome browser and I open admin url into my firefox window and it worked. Though in the firefox the url http://localhost/magento/index.php/admin did not work, but url http://127.0.0.1/magento/index.php/admin worked fine.
Second Solution
Go to your Magento Installation directory and navigate to the app/code/core/Mage/Core/Model/Session/Abstract/Varien.php file.
Find the code:
session_set_cookie_params(
$this->getCookie()->getLifetime(),
$this->getCookie()->getPath(),
$this->getCookie()->getDomain(),
$this->getCookie()->isSecure(),
$this->getCookie()->getHttponly()
);
and replace with
session_set_cookie_params(
$this->getCookie()->getLifetime(),
$this->getCookie()->getPath()
//$this->getCookie()->getDomain(),
//$this->getCookie()->isSecure(),
//$this->getCookie()->getHttponly()
);
After this save the file and I think it will work now.