Requesting For user Login.

I have a domained 2000 (SP4 + Latest Patches) server that is running
masterfile, this is a intranet site. The IIS Site authenticates through
Intragrated Authentication.

recently the site has been asking for a user name and password. Now this
only happen for some of the users but not all.

Now i was wondering what would cuase this, it wasn't doing it and all of a
sudden its asking for a user name and password.

Now the only recent changes that have been made to our network has been the
introduction of 2 server 2k3 machines, 1 hosting Exchange 2003 and one as a
AD Master (It runs all 5 master roles.).

i have tested the two new servers and they all come up fine, in both
autheniticating and been authenicated.

Any help would be appreciated.