Accessing a network file, 'Access to the path is denied'

In my ASP.NET C# I am trying to load a text file from a network drive and put the contents into a TextBox using the following code.
string path = @"c:\temp\testfile.txt";
System.IO.File.Copy(path,Server.MapPath("testfile3.txt"));
Problem I'm having is the following error appears when I attempt to load the file.
'Access to the path is denied'
I've tried various methods to grant access to this file but with no joy, I am using IIS 6 with Visual Studio 2008.
Any suggestions/comments welcome
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