Anonymous IPP Printing Problems

Have got a W2K Domain set-up and IPP installed on our File & Print Server.
This works for all Domain Users but I have a WiFi Firewall and visitors who
wish to be able to print.

Obviously, the visitors are not part of our Domain and simply get an IP
address issued. I have set-up the following ports on the WiFi Firewall:

631
9100
515
80

Problem is that when a visitor clicks the 'connect' text from within the IPP
web page it takes 30 seconds to try and install the drivers and produces the
following error:

'Printer Installation Failed. The printer driver is unknown'

Does anyone have any ideas?

Thanks