Fronpage Access

We have a ISA server in the internal domain(win2000AD). On the extenal, we got our web site in a different Domain (win2000AD). When using frontpage not going throught the ISA the password is accepted,
But if you go via ISA then the login box keep popping up everytime we login?
Any suggestion, we stock and we want user to go via ISA not direct for scurity reason. Thanks