INTRANET problem

i have a winnt PDC and a BDC.
my IIS is installed in the BDC.
i am having intranet also.
using intranet webpages when i browse to a certain directory/file in the BDC server it throws out a popup asking for username/pw/domain and if valid can viewthe page.
but it is different in the folder of the PDC when I try to access as it staight away does not validate but gives a Error page.
why is is so even though both are in the same domain and having the same SAM ?