Error when Client posting file to Win 2003's IIS

[Rabbit]

Dear all,

My application contains 2 parts(Server and Client), the server side is
having a
ASP.net page as listener for client side to post file, the main coding to
receive posting file as following:
----------------------------Listener major
coding--------------------------------------------
Line 1 Dim strAppPath As String = HttpRuntime.AppDomainAppPath

Line 2 For Each tmpfile In Request.Files.AllKeys
Line 3 Uploadfile = Request.Files(tmpfile)
Line 4 Uploadfile.SaveAs(strAppPath & "working\" & Uploadfile.FileName)
Line 5 Next tmpfile
----------------------------------------------------------------------------------------

The Client side is a .net window form application which allows user to post
a list of files(.xml, .jpg) to the server, the main coding as following:
----------------------------Sender major
coding------------------------------------
Dim myWebClient As New WebClient
Dim responseArray As Byte()
Dim intCnt As Integer, strPictureFile
Dim strURI As String = "http://www.mywebserve.com/Uploader.aspx"

responseArray = myWebClient.UploadFile(strURI, "POST",
"c:\uploadfiles\testing.jpg")
----------------------------------------------------------------------------------------

This coding was working perfect on WinXP and Win2000 server platform, but
now when I use the same architecture on our new Win 2003 Server, the server
always
return error on Line 4, saying (500) Internal Server Error

So I wonder anyway to find out what was the problem of setting ASP.Net on
Win2003 Server?
I have already tried following:
*) Restarting ASP.Net (with "aspnet_regiis.exe -i")
*) Check the web site has same configure as my previous production server
(Win2k)
*) Installed Win 2003 service pack 1

Best Regards,
Keith