always ask me for a password

I installed sharepoint on my server with IIS 6.0. Some users are having
problem logging on. although it is setup to use windows authentication, some
users are asked to logon when they try to access share point while others
are not asked. Other users can not logon after they change their password.
They get error 550. I will have to change it back to the old one. I am using
windows secure authentication (https) with the certificate on my domain
server.

I am using the same configuration on another server (https) for my OWA and I
do not have this problem.

Any idea?

I am using Windows 2003 enterprise server. Share point 2003 and ISA 2004. I
am hosting everything locally.