No Network after modem connects on dialup?

I have been seeing a LOT of computers coming into the
office lately with the same problem: They are dialup
users, and they can dial in just fine, and it says they
are connected and the system tray shows the connection
icon. And I've checked most of the obvious settings on the
machines--they should be able to connect just fine.

Even the routes look fine. I can use nslookup and resolve
IPs, so I know it's not a DNS problem.

But IE and Outlook Express can't seem to find a network
connection. And Norton Antivirus says the network is not
properly configured.

Any ideas by what I've said so far? I can probably pull a
bunch of specifics and copy/paste them to a text file and
post them as well, if you want more information. My first
guess is something wrong in the services. There's no
firewall, no proxy, etc.

Oh, these are XP machines. I find it convenient that this
problem crops up at the same time as Sasser...