Make XP logon think LAN's disconnected

When our users need to access our network from a remote site, they
plug their laptops into a small LAN consisting of a switch and an ISDN
router, which then connects them to the office network back at base.
This is all running in an XP/W2K3/AD environment.

The problem we've got is that when someone on a laptop logs in, XP
assumes that the 100Mbps link it's got to the switch runs all the way
to the server at the other end of the ISDN link, which it obviously
doesn't, and then tries to go through the full authentication, group
policy, etc.. routines which takes forever over the shared ISDN link.

What I'm after is some way of making XP think it's got no connection
to the server when logging on (forcing it to use things like its
locally cached profile) and not touch the network, but still have
TCP/IP connectivity over the ISDN link for applications such as Lotus
Notes.

Is there any way of doing this?