Securtiy concerns about IIS websites' access rights - Restrict them not to be able to read c:\, etc

I have some security concerns about my IIS websites.
From PHP-files (FastCGI) on our server the scripts are able to read my system's root directory (c:\), but even list the content of c:\windows. What is more troublesome that I can even create directories in the root like this: mkdir('c:\asdasd');, and it works without any problems; I can even read the other websites' directories (so actually maybe I could map the whole system's directory tree from a PHP script). I think that's a huge security problem. Noone should be able to read or write c:\, or other hosted website's content.

I know there's the open_basedir directive in PHP, but I think setting this directive doesn't solve the problem as I could also use any other server languages to access the same mentioned directories.

The directory tree of my websites look like this:

c:\net\web\php\
   |- website1
      |- other
      |- htdocs
   |- website2
      |- other
      |- htdocs
   |- website3
      |- other
      |- htdocs
   |- ...

The websites' source files are in the htdocs directory, so this is the "root" directory of these websites.
I just want to enable read/write permissions to the first parent directory of the website, like for example c:\net\web\php\website1 (so e.g. "c:\net\web\php\website1\other" could be written, but c:\net\web\php\ should not be).

How can I restrict the access this way?

Thanks!