Disable Network Prompt

Hi! I have set-up IIS/OWA with Exchange Server 5.5 SP3 together on a computer, enabling NTLM authentication in IIS. Our users access OWA via the internet. We added some code on logon.asp so that the user's mailbox name is automatically entered but my problem is the network prompt (Name, Password, Domain) comes up the first time. Can we avoid this first time prompt? We want our users not to enter their mailbox name/pwd on the prompt on their first logon.