A few links we suggest for help with securing your Joomla sites:
http://docs.joomla.org/Security
http://developer.joomla.org/security.html
http://docs.joomla.org/Security_Checklist/Getting_Started
Checklist If You've Been Hacked:
http://docs.joomla.org/Security_Checklist/You_have_been_hacked_or_defaced
Remember, the best way to stay secure is to keep your site updated.