Access a public folder from an application deployed in IIS

I have a problem trying to access a file from a shared folder on the local network.
When I run the application in Visual Studio, it works fine and I can upload the file, but when I deploy the application in IIS, it just doesn't work.
I do not understand why, because I give full permissions to all users, including NETWORK SERVICE and IIS_USR.
I am currently using my computer to verify this. It has Windows 7 and works with IIS 7.5. The shared folder I'm trying to access is a Windows Vista installation that does not have IIS installed.

Code for downloading the file:

protected void button_Click(object sender, EventArgs e)
{
    Response.Clear();
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=S10.png");
    Response.WriteFile(@"\\192.168.1.82\Machine\file.png");
    Response.End();
}

and this gives me the following error:

: -. .

: System.UnauthorizedAccessException: Acesso negado ao caminho '\ 192.168.1.82\Machine\file.png'.

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