Kim:
This is going to sound like an excuse, but it isn't our job to make sure that your server and hosting environment is secure, we don't control it.
Kim is right about the server and hosting environment. I would suggest though that there are some PCI requirements that pertain to application development, and that they do apply:
6.2 Is the software and application development process based on an industry best practice and is information security included throughout the software development life cycle (SDLC) process?
6.5 Were guidelines commonly accepted by the security community (such as Open Web Application Security Project group (https://www.owasp.org) taken into account in the development of Web applications?
6.6 When authenticating over the Internet, is the application designed to prevent malicious users from trying to determine existing user accounts?
6.7 Is sensitive cardholder data stored in cookies secured or encrypted?
6.8 Are controls implemented on the servers side to prevent SQL injection and other bypassing of client side-input controls.
Perhaps that last is more relevant to the server environment. Of course, Zen Cart is Open Source so anyone can modify it, and modifications would have to comply with the above guidelines as well.
Food for thought.