I have downloaded and added this very simple, one file, php web file explorer system(called Indexer) to my XAMPP server.
My XAMMP server is on my C: drive, but I want In
Let's take a look at that script:
$Root = realpath("."); // define the directory the index should be created for (can also be located outside the webserver root)
$AllowDownload = TRUE; // enclose file items with the anchor-tag (only makes sense when the files are in the webserver root)
$WebServerPath = dirname(getenv("SCRIPT_NAME")); // path where the indexed files can be accessed via a http URL (only required when $AllowDownload is TRUE)
Notice "only makes sense when the files are in the webserver root" and "path where the indexed files can be accessed via a http URL". Which indicates that this script was not designed to be able to download files that are outside the web server root dir.
However, you could modify this script to be able to do that in the way that styu has noted in his answer. You could then send your changes to the author of the script.
BTW, I tested this on my own server.