XP client domain auth. on NT4 server

Hi guys, hope all is well?.

A bit of a puzzler for you........

XP Workstation & NT4 Server

We have two users who share the same machine. They
authenticate through us by logging onto the CORPORATE
domain (NT 4.0 Server domain). Up until last Monday both
users were able to log onto the machine and authenticate
to the domain just fine. Now, the one user, Eric, is able
to log onto the machine as always and authenticate to the
domain just fine, but the other user, Gary, is not.
Whenever we try to log Gary in we get the message that the
user cannot log in because the domain is not available.
Now here is where it gets really weird...

1. I can sit at my desk (remote site) and log on as Gary
and authenticate to the domain using an XP machine. When I
contacted a user at our sub with Admin privileges, he
could not log in as the user and got the domain not
available message.

2. I created a completely new user account and changed the
password. Again, I can log in as the user here, but no
one at the sub can log in as the user on his machine or
any other.

3. I cannot log on through the local user account or the
local Administrator account.

So we are stumped. Eric can log in, but Gary cannot.

Any help/resources would be sincerely appreciated.

Thankyou............