could be the DNS issue. this may help. quoted from
http://www.howtonetworking.com/winslow.htm
Why is XP running slower while logon 1
When I login into a Windows 2000 active directory from a new xp pro machine
the login in is extremely slow. After logged on, everything works fine. But
other windows versions don't have this problem.
A: This is almost like name resolution issue. Comparing with previous
versions, XP is heavily dependent on DNS to find DC. If the DNS is
configured incorrectly, XP will take longer time waiting for it to timeout
before it tries using NetBIOS. Make sure
1) The DNS setup correct and have the server information.
2) XP clients can find the DNS server.
3) XP machine's DNS server is pointed to your internal DNS server rather
than your ISP's DNS server.
4) Make sure no errors on logon scripts or GPO's that could be causing
the delay.
5) Check any errors on event viewer.
6) Disable NetBIOS on the interfaces that client will not use.
--
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