Using Internet Explorer for FTP access

I have a group of workstations at a client site that access an IIS NT4 System to view and sometimes download (rarely)files. The client site is connected via a dedicated ISDN link and is behind one or two firewalls. The client (NT4 Wrksta) is able toftp using the command line (DOS). When the try to connect via IE they get user authentication errors and event ID100 in the system log on the IIS system. Any ideas? Thanks in Advance…….