Windows XP Pro looses network authentication

I have a problem that I have been trying to resolve for
some time but no success yet. We have a corporate network
consisting of several pcs and several servers. The
operating systems used are Windows 95, 98, ME, 2000, NT4
and Windows 2000 servers. The new pcs that we have bought
came with Windows XP Pro. These pcs login to a Windows
2000 Advanced server. Here is the issue.
After logging to the Windows 2000 domain, it seems
like the Windows XP Pro pc looses its network
authentication after a hour or so. After an hour I can
browse the Windows 2000 server but I can't browse any of
the NT 4 servers. The NT 4 server are also a part of
Windows 2000 domain. When I try to browse the NT 4
server, my XP sits there for a couple of minutes, then
prompts with a login box. I have tried to use my domain
login id and password but it does not accepts that. Gives
a message to the effect that login has already been tried
using that id and password. Windows 98 pcs does not have
that problem, they can browse any server any time, it is
only Windows XP Pro pcs. I have checked folder rights,
Active Directory and everything else that I can think of,
all seems to be correct. I have searched Microsoft's web
site and the web several times for any solutions. Tried
everything that I have found but it is not resolved. I
would highly appreciate any help or tips/pointers from
readers.Thank you in advance.