Accessing a file on the client side

Hi All,

 I have a web application that is mainly (90%) used from within my network. I want this asp.net app to access a text file on the client side in which I want to store the users AD domain and username. I can understand why I cant do that on a 'normal' website without the users express permision but this is internal and i am the network admin. I have tried accessing the user name in various ways (System.Security.Principal) etc but as soon as i put it on my server (internal to network) it cant see that information.

 Any ideas?
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