Simple: Find out Joomla 1.5 version on your site
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.