Can't login magento Admin with right username and password

http://stackoverflow.com/questions/2176195/cant-log-in-to-magento-admin

If you use sample data:

Change: app/etc/local.xml line 55

<session_save><![CDATA[files]]></session_save>

replace

<session_save><![CDATA[db]]></session_save>

Then: Clear browser cache