WinXP Pro disconnecting from Win NT4 Server

I have three Win XP Pro clients that log onto a network
consisting of two servers one which is NT4 and another
which is Win2000.
All XP clients logon successfully to both servers, however
if the client PC's are not used for 30 minutes or so they
seem to disconnect from the NT4, but it is still fine with
the Win2000.
I've check all power management setting etc and they are
all fine. Please could anyone shad any more light as to
what could wrong. Thanks.