the scandir function is defined by php 5:
function.scandir.php
create a new blank php file and add the following code to it:
<?php phpinfo(); ?>
this will tell you about your php installation if it is not php 5, you will need to have the host update your php version.
if it running php 5 and you are getting this message, you should contact the host to determine why the scandir function has been disabled.