Once you rename the file you must make a change in the config file to support the new name you gave the admin.php file.
Below is a step by step on how to accomplish this.
1. Rename the 'admin.php' right on the server to whatever you like example: admin8765.php
2. Download 'config.php' from your server and open in notepad.
3. Change the following line in 'config.php' to refelect the changes you made:
$admin_index = 'admin.php';
to
$admin_index = 'admin8765.php';
4. Upload 'config.php'
5. Go to www.yourdomain.com/[CS-CART]/yournewadminpage.php to test.