Gateway times out after login to SBS 2K3 server?

I posted this in the general Windows networking forum, I apologize, I
didn't find this group until afterward:

This is a new one on me.

Small business network, with an SBS Win2K3 server. Single subnet,
cable modem, pretty basic. All client computers are fed DHCP/DNS from
the SBS server, and non-local DNS queries are forwarded on to ISP's DNS
servers.

This network has been running about 18 months on the exact same
configuration. No new hardware or software has been installed in the
two months prior to this problem appearing - aside from Microsoft
Security patches.

At some point over this past weekend, all clients on the network (and
also the server) are unable to access the default gateway _after_ a
login to the SBS. Any administrator login we've tried does it.
_Prior_ to login (reboot the server and let it sit at the login screen)
everything on the network operates just fine.

Cable modem->router_points_to_internal_SBS_IP

After a login, within a few seconds, pings from inside the LAN to the
default gateway (all machines are 192.168.1.x with gateway 192.168.1.1
which is the router) time out, and pings from outside the network to
the public IP address time out, whereas before login pings to both are
fine. The DNS server shuts down within a minute or two, and of course
takes the client machines with it.

Adding to my confusion, if I establish a remote desktop connection to
the server using either Remote Desktop or a remote access program
called Remote Administrator _before_ I login, the connection continues
to run for about half an hour after the pings die and the DNS server
dies and everything loses connectivity. The remote session eventually
dies, too, but much, much later.

The ISP says nothing is wrong on their end. I've rebooted everything
on the network that can be rebooted, I've replaced cabling, swapped
ports on the hub, and even replaced the network card. I've removed
everything from the startup group, and the only things starting up
shown in msconfig aside from services are the raid monitor and a few
SBS pieces. I've run antivirus check, adware and malware checks.

The server runs Exchange, shares files, hosts Symantec System Center,
and runs an online backup. There is nothing else installed on it.

Finally, if I log off of the account on the server connectivity to the
gateway is not restored. However, if I shut down the server, at some
point in the shutdown process external and internal pings to the
gateway do begin responding before the server shuts down entirely.

So, in short, whatever is happening happens only after someone logs in
to the server, and it affects client machines as well as the server
itself.

The only error in any of the event logs is an event id 113 citing 1168
when the DNS server cannot update, which is thrown just prior to the
DNS server shutting down. I'm unable so far to find anything
discussing this event that sounds like my issue.

Anyone have any ideas at all?

Note that everything local continues to work just fine before or after
login, whether the gateway issue is present or not. All files and
folders are accessible, names are resolvable, the Exchange server
continues to function (with the exception that it cannot send outgoing
email when it cannot resolve external names via DNS when the gateway is
inaccessible), and for all apparent purposes everything is behaving
normally. I'm at a loss.

.

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