Network Authentication Timeout

I have a small Win 2003 Server domain, workstations XP Pro SP2. It appears
that the authentication of my users in the domain expires after a certain
amount of time, at the most a few hours. For example, if a user logs on at 8
AM and leaves the session open until 6 PM without using the network, they
can no longer access resources in the domain 'Access denied... possible
security breech'... etc. If the user then logs out and logs back in
immediately, access is restored. Why is this happening? How can I prevent
this?

Thanks,
MN

See my other post two message threads away.

--
Todd J Heron, MCSE
Windows Server 2003/2000/NT; CCA
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