Magento Admin Login not working on Localhost.

Here is the simple fix to override this problem.
This is for Magento v1.3.2.4 :

Open this file,
app/code/core/Mage/Core/Model/Session/Abstract/Varien.php
and find the    session_set_cookie_params(line-78) and replace your code with this ;

session_set_cookie_params(
$this->getCookie()->getLifetime(),
$this->getCookie()->getPath()
//$this->getCookie()->getDomain(),
//$this->getCookie()->isSecure(),
//$this->getCookie()->getHttponly()  

also remove the comma from end of getPath().

This is for Magento v1.4.0.1 :Open up Varien.php at magentoappcodecoreMageCoreModelSessionAbstract and find the following snippet starting at line 77:

// session cookie params
$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();
}
  Now comment out the relevant parts like so:// session cookie params
$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();
}*/