Notwithstanding that your host is clearly not being helpful, there are a few things you can do in response to the problem:

1. Follow the instructions provided by your host and the error message. If you follow their instructions, you should be able to see the real error message behind the problem, which will lead to more potential for a useful answer here.

2. Since IIS servers don't like the .htaccess files used by the more versatile and reliable Apache web engine, you might find some success in deleting the various .htaccess files which were included with your Zen Cart files. You'll need to work with your host on ways to provide the security protections that the .htaccess settings normally provide. Of course, that depends on your host being helpful ...