Direct Access to this location is not allowed. Error

I have looked thru the forum and FAQ's and non really mention this.

I am running Joomla 1.5, and had tested the local SEF component prior to purchasing the SEF Advance 2008 (as suggested in the documentation)

Once I installed it, and activated it, when I goto my site I get:

"Direct Access to this location is not allowed."  for any page I try to goto, including the homepage.

Any advice ??

This error is probably generated by a 3rd party component that utilitize sef_ext.php plugin for Joomla! 1.0. Locate the component in question and uninstall it or delete/rename its sef_ext.php file. Alternatively, activate the legacy plugin in your Joomla! 1.5 plugin administration.