The default installation of Alfresco requires installing software downloaded from the SourceForge project location (http://sourceforge.net/project/showfiles.php?group_id=143373). Now Alfresco is hosting its own enterprise download area. You can also download this from http://www.alfresco.com/products/ecm/enttrial/. Select the download package. And you will be asked for the user name and password of the Alfresco content community.
To install and run Alfresco, you need at least 1 GB disk space and at least 1 GB RAM on the desktop or server.