executing javascript function causes unwanted directory listing

I have an ASP.NET app (c# codebehind) in which I alow the user to browse for a file using <input type=file>. Clicking on another link calls a javascript function which is to open the designated file and read its contents into a textbox. I am pretty sure I can do all of that without dificulty... BUT for reasons that I don't understand, simply calling this javascript function will result in a directory listing (or attempted directory listing)... In fact, I even tried putting a return statement at the very beginning of the function so that none of the other code will execute, and even that causes this stupid directory listing...

How can I make this directory listing go away???

Here is a snippet of the HTML code:

Code:

---

<P>Select a file<br>
<input id="File1" type="file" size="40" name="File1">&nbsp;
<a href="" onclick="Preview()">Preview the file contents</a></P>

<P><STRONG>File contents:</STRONG><BR>
<asp:textbox id="TextBox3" runat="server" Height="74px" Width="554px"></asp:textbox></P>

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Here is the source of the javascript function:

Code:

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function Preview()
{
        try
        {
                // get the filename...
                var file_val = eval("document.Form1.File1");
                var strFileName = file_val.value;

                // open the file, read it, put it in the text box...
                var fso = new ActiveXObject("Scripting.FileSystemObject");
                var f = fso.OpenTextFile(strFileName, 1);
                var str = f.ReadAll();
                //alert(str);
                document.Form1.TextBox3.value = str;
                                       
                f.Close();
        }
        catch (Exception)
        {
                alert("Cannot open file :( " + Exception.message);
        }
}

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Is the actual script being run on the client-side or server-side?