Exchange 2003 OWA users can’t login

I have moved from Exchange 2000 to Exchange 2003
for the most part it went well but when users trye to access the OWA it will ask for thier username and password, when you enter them in and click ok it comeme back with the the mail.domain.com\username and it just keeps coming back until it displays Error: Access is Denied.
the address is mail.domainname.com\exchange.
Now an odd discovery I just tryed with Firefox and it worked????
IE 6 no
Firefox works
What could it be????