Need to find out which version of Joomla 1.5 is being used on your site (1.5.26 being the latest).
Here is how to find out your Joomla 1.5 version:
1. Connect to your Joomla installation via FTP (or just go to your installation folder)
2. Open libraries\joomla\version.php
3. On row 29 find variable DEV_LEVEL which is showing your current version of Joomla 1.5.
If yours is less than 26, please perform your updates. Download Joomla 1.5 latest from Joomla 1.5.26 upgrade packages page.