f the domain is not a true domain then this problem occurs. Its like if you use your localhost to run magento store,then you will face this problem.
You can overcome this by browsing using ur ip address or you can remove these three line from the following page.
app\code\core\Mage\Core\Model\Session\Abstract\varien.php
$this->getCookie()->getDomain(),
$this->getCookie()->isSecure(),
$this->getCookie()->getHttponly()
Comment these lines line (may be at line 78…);
Hope magento admin will work now on your localhost with the following url.
http://localhost/magento/admin/
Just try out and play with your stores……