Switch 3com and "The system cannot log you on now because the domain "name" is not available."

Hello, everybody.

I'm seeng the following message on some computers: The system cannot
log you on now because the domain "name" is not available. If you let
the system run for a couple of minutes, the system accepts your log in.

This happens only the first time my users try to log on in some of my
public computers.

I ran a test and discovered that only those computers connected to a
3Com SuperStack 3300 XM switch behave that way. Computers connected to
a hub or a simple D-Link switch work correctly.

I analized the packets comming from different PCs and noticed that the
ones connected to the 3Com switch start talking quite some time after
the other ones.

Thus, I assumed it was a problem with stpFastStart, so I enabled it in
some ports. Problem HAS NOT gone away. If I create a new account (to
avoid cached credentials) and try shutdown the PC and power it up again
after some minutes and try logging in again, it gives the message
again.

I have tried disabling STP. I'm pretty sure I don't have ANY loops in
my networks.

The clients are Windows XP Professional and the server is an NT4
grandpa. It APPEARS that Windows 2000 computers do not have that
problem.

Any ideas?

Thanks.