asp.net write permission on w2k domain controller

[Stephen Witter]

I am using sql server reporting services on a domain controller. It
typically uses the aspnet user account, but I had to switch to
IWAM_machine account due to an issue with w2k sp4. Everything works
great, however, when I try to write or stream a file to a directory in
my wwwroot folder using impersonation I get an access denied error
(400). My question is does the aspnet account (which doesn't exist on
a domain controller) still get passed when using impersonation, or
does the IWAM account get passed. Here is the code:

Try
Dim TempDir as string =
Server.mappath("/TempReports/tempReport." &
SelectExtension)
Dim stream As FileStream = File.Create(TempDir, result.Length)
stream.Write(result, 0, result.Length)
stream.Close()
Dim sFileName as string =
"http://MyWebServer/TempReports/tempReport." & SelectExtension
response.redirect(sFileName)
Catch ex As Exception
Response.write(ex.Message)
End Try

It bombs before the file is created. Any suggestions?