I use this site as a reference:
http://richhewlett.com/2011/02/15/getting-a-users-username-in-asp-net/
(reposted summary in case site goes down)
Scenario 1: Anonymous Authentication in IIS with impersonation off.
HttpContext.Current.Request.LogonUserIdentity.Name COMPUTER1\IUSR_COMPUTER1
HttpContext.Current.Request.IsAuthenticated False
HttpContext.Current.User.Identity.Name -
System.Environment.UserName ASPNET
Security.Principal.WindowsIdentity.GetCurrent().Name COMPUTER1\ASPNET
Scenario 2: Windows Authentication in IIS, impersonation off.
HttpContext.Current.Request.LogonUserIdentity.Name MYDOMAIN\USER1
HttpContext.Current.Request.IsAuthenticated True
HttpContext.Current.User.Identity.Name MYDOMAIN\USER1
System.Environment.UserName ASPNET
Security.Principal.WindowsIdentity.GetCurrent().Name COMPUTER1\ASPNET
Scenario 3: Anonymous Authentication in IIS, impersonation on
HttpContext.Current.Request.LogonUserIdentity.Name COMPUTER1\IUSR_COMPUTER1
HttpContext.Current.Request.IsAuthenticated False
HttpContext.Current.User.Identity.Name -
System.Environment.UserName IUSR_COMPUTER1
Security.Principal.WindowsIdentity.GetCurrent().Name COMPUTER1\IUSR_COMPUTER1
Scenario 4: Windows Authentication in IIS, impersonation on
HttpContext.Current.Request.LogonUserIdentity.Name MYDOMAIN\USER1
HttpContext.Current.Request.IsAuthenticated True
HttpContext.Current.User.Identity.Name MYDOMAIN\USER1
System.Environment.UserName USER1
Security.Principal.WindowsIdentity.GetCurrent().Name MYDOMAIN\USER1