Only administrators can logon to OWA 2003

I have searched the internet and found supposed solutions to this problem previously, including make sure that user have log on locally rights to the server but they didnt fix the problem so im hoping someone else here has another idea.
Here goes.

I have 2 servers, one is the backend and one front end server both running 2003 server with 2003 exchange.
All clients are Xp Pro with IE 6 SP1.

The administrator or domain admin groups on the network can access the owa simply by going to the correct url, and NTLM (as was) deals with the logon.

However, any other user group is prompted with a popup login box, even when entering the account details does not allow you to logon. Unless you type an admin user/pass in which case you get the admins email.

Any ideas gratefully received.

Thanks in advance.