Extremely Slow for User Logon

Do you run Win2K Servers with Active Directory enabled, or are you still using NT4 PDC/BDCs?
If you still have the NT4 machines, then that is why. The XP boxes use DNS as the primary method for finiding a domain controller to authenticate with. After while, they fall down to using a WINS server and/or network broadcast.
You can speed this up with the LMHOSTS file on the XP boxes. Set them up for authentication.

Please read this Technet article:
http://support.microsoft.com/default.aspx?scid=KB;en-us;q180094
(please remove any spaces)

hope this helps

If its an AD domain you are in, it could be caused by
having several GPOs to process.

cheers
Marc

This often happens if the XP machines are configured to use your ISPs DNS servers rather than your internal Active Directory one.

Make DHCP register all leases in DNS too.