Windows XP connecting to Web Printing on 2000 Server

I have a 2000 server running IIS Web Printing. All was
fine, I was able to get the XP clients to connect to
printer by forcing them to use the Windows 2000 drivers,
but something has changed! Now when I ask an XP machine
to connect to a share printer using the Web Page install
I get a "Configure Internet Port" dialogue box asking me
to log in again. No matter what I use at this point the
login fails and so does the printer installation. I can
however, connect to the server using Windows networking
and install the shared printers that way just by double
clicking on them. The same login credentials work ok in
this case, but not using the Web Page. Any help would be
greatfully received. Steve.