Unable To Write File From ASPX Page. What Am I Missing?

I have a folder on webserverwwwroot, called staticContent. The folder
has several message of the day text files, such as RepMotd.text. I have
an admin page, AdminMotd.aspx, that allows a user to view and edit the
contents of the files.

Webserver is running Win Server 2003 64-bit and IIS 6.0. The development
machine is Win XP, using Visual Studio 2005 (thus ASP.Net 2.0.)

In IIS, staticContent folder has Write checked and execute permission is
set to Script Only . In NTSF, the folder grants full permissions to
Network System.

When I try to overwrite an existing file in staticContent, or try to
create a new file, I get a System.UnauthorizedAccessException. The
Application error log on webserver notes that the error is on the page
I am trying to run, that my user name is what I logged in as, that I am
authenticated, and that the thread account name is NT AUTHORITYNETWORK
SYSTEM. Elsewhere, I am able to read these same files and insert them
dynamically into the web page.

This is the code that I am trying to run in AdminMotd.aspx, cut down to
the basics. If it makes any difference, the code is run "in page" and
not as part of a separate code-behind file.

******************
<script runat="server">
Protected Sub MotdChange(ByVal ThisFile As String, ByVal ThisText As
String)
Dim sw As StreamWriter = New StreamWriter(StaticRoot + ThisFile,
False)
sw.Write(ThisText)
sw.Close()
End Sub

Protected Sub RepMotdChange_Click(Byval sender As Object, ByVal e As
System.EventArgs)
MotdChange("RepMotd.text", RepMotdText.Text)
End Sub
</script>

<asp:Content ...>
<asp:TextBox ID="RepMotdText" runat="server"></asp:TextBox>
<asp:Button ID="RepMotdChange" runat="server" />
</asp:Content>
********************

I have traced that the error occurs when New StreamWriter is called,
rather than when actually trying to write to the file. Obviously I'm
missing something, but danged if I can find it.