Symptom: Your OSCommerce installation gives you the following message at the top of your oscommerce page:

Warning: I am able to write to the configuration file: E:\inetpub\vhosts\domain.com\httpdocs\oscommerce\catalog/includes/configure.php. This is a potential security risk - please set the right user permissions on this file.

Solution: Once your OScommerce installation is completed, you need to mark the configure.php file as READ-ONLY.  Plesk does not provide a function for doing this, but the support team will gladly make this change for you - simply submit a support request giving the path to the configure.php, and asking to set the READ ONLY attribute on the file - and reference this KB article.