If all you want to do is show the client their own files, you can do that with a simple link:
<a href="file://c:/windows/temp">Show files</a>
Not sure I have the syntax right, but it doable. Joke sites used to use such links to claim they could 'see your hard-drive' and some shady security sites used it as a ruse to scare people into purchasing their products.
I'll try it with VBCode to see if it works...
[url="file://c:/windows/temp"]Show files[/url]
Seems to work. Of course, you will need to use a different link depending on what operating system the user migh have if you are targetting a broad audience.
If you want to do something more, I think you would need to install a client side application of some sort, or maybe use activeX controls (there always seems to be a way to break the rules with activeX)