To be honest, I'm not sure if this is even possible--at least, without
extensive work.
ASP.NET code executes on the server, and as such, that is all it has
access to.
In addition, JavaScript is restricted from accessing the file system,
so that rules that out.
The only way I could imagine being able to access the client's file
system is to create an ActiveX control that the clients would have to
download and install, and that code would have to be trusted on the
client machine.
You may be better off to re-evaluate the architecture of your
application--does it really need this functionality? If the answer is
yes, you should probably look into developing a windows forms/smart
client.
What exactly are you trying to do, anyway? Why would a user go to your
website to navigate the files on his local machine?