If you find yourself unable to login to your newley-installed Magento admin area, you may want to try this:
Open the file Varien.php in a text editor. You will find the file in app > code > core > Mage > Core > Model > Session > Abstract > Varien.php.
Go to somewhere around line 75 and comment out these lines:
/* set session cookie params
Save the file and try logging in again. (This tip came off the Magento forum and has helped me more than once so I thought I’d share it!)