Capture Windows userid with Anonymous accesss Enabled

Hi,
I need to capture the windows user id as part of my web application. But I have to set the Anonymous access Enabled on the IIS. Because we don't want to prompt the users for their login credentials. I would like to see who inserted/ updated the data. This is for an intranet application in C# .Net 3.5

I have tried the following,
Request.ServerVariables["LOGON_USER"] which returns Blank
Page.User.Identity.Name which returns Blank
System.Environment.UserName which returns 'NETWORK SERVICE'
 System.Security.Principal.WindowsIdentity.GetCurrent().Name which returns 'NETWORK SERVICE'

Thanks,
HT