Win98 logon

I have a network with some Win98 Workstations. The AD Server (Win2003)
is either the internet provider. I would like to block all connections
from the network to the W2003 Server to use a Proxy service. I will
unblock sends direct to the W2003 Server IP to allow file access and
anothers. The problem is how to configure the W2003 to accept the logon
of the W98 (today, the W98 display a message like server not found for
authentication).

Thanks,

Max