No domain server error when logging into domain

Hi

I've got some windows98 machines which are getting the
following error when trying to log remotely via a VPN to
my domain controller: "No domain server was available to
validate your password. You may not be able to gain
access to some network resources." This is happening with
only some of my Win98 machines within the same location.
I've checked IP's and WIN's has been setup at the router
level so no need to put it in statically. There are no
TCP/IP settings put in statically, everything gets issued
by the router.So, basically what we've done is put the
computer on the domain, specify the domain name, and
reboot. Please provide some urgent advice.

Thanks
Regards