Long term you should really have the hosting provider configure the server to run a newer version of PHP (5.2 and older are ancient in computer years). Once the account has been upgraded you can re-install Zen Cart 1.5.4.
In order to run the Zen Cart installer, you may need to do
one of the following (after uploading a clean copy of zc_install from the distribution zip matching the installed version of Zen Cart):
- Disable your alternative URLs plugin.
- Add an rule to the .htaccess to not rewrite "zc_install".
- Manually type in the full path (/zc_install/index.php).