Getting a SharePoint user

Hi all, I have a script which writes data to a sharepoint list (using asp.net VB). I'm using the following for credentials:
Dim req as HttpWebRequest = HttpWebRequest.Create("https://mysharepoint/_vti_bin/Lists.asmx")
req.UseDefaultCredentials = true

But the defult credentials show the Modified By user as IUSR_NAME I would like the credentials to be those of HttpContext.Current.User.Identity.Name.ToString() - If i use
response.write(HttpContext.Current.User.Identity.Name.ToString())
The desired user is shown. Is there a way to set req.Credentials to the httpcontext user instead? thanks
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