IIS6 Integrated Windows Authentication

i'm having a problem using Windows Authentication for a program on IIS6. this works fine in IIS5,so there's something i'm missing somewhere.
bascially when the page loads i'd like to extract the domain and username.
ususally i use :
System.Security.Principal.WindowsIdentity.GetCurrent.Name

and set Directory Security checking only "Integrated Windows Authentication"
for some reason IIS is returning: NT AUTHORITY\NETWORK SERVICE
i've unchecked everything else and can't understand why this is so different in 6.0.
any help or guidance is appreciated.
thanks again
rik