Its a long shot but you could give this a try, when xp tries to connect to a samba domain controller it kind of gets upset with security channels.
On the xp machine just edit the registry with the following key:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Netlogon\Parameters]
"requiresignorseal"=dword:00000000
fingers crossed that should help.
If not have a nosey in your windows event logs and see if there is owt under there
DJ