Outlook Web Access Exchange Server Logon

I have a Windows NT 4.0 box configured to work as a web server offering Outlook Web Access to our Exchange 5.5 server. I would like to allow users to log on to their web mailbox WITHOUT having to preceed their username with domain name (e.g. domain/username). I would simply like to allow them to type their username and password. Also, I have a single domain, single Exchange box setup.

Thanks,
Kurt