Fatal error after installation

I've just tried to install Omeka but I'm getting this error message when I go to the url. Any ideas why?

Warning: require_once(Zend/Loader.php) [function.require-once]: failed to open stream: No such file or directory in /home/londonex/public_html/application/libraries/Zend/Controller/Front.php on line 23

Fatal error: require_once() [function.require]: Failed opening required 'Zend/Loader.php' (include_path='/home/londonex/public_html/application/libraries:/home/londonex/public_html/application/models') in /home/londonex/public_html/application/libraries/Zend/Controller/Front.php on line 23

Are you sure that you've properly uploaded all of the files in Omeka's core? It looks like Omeka is trying to load files that don't exist.

In addition, can you double-check that you have included the .htaccess files in both the root of your application, and the admin directory?