This should be performed on the XO client prior to joining you AD domain.
But..if you have already joined it, then go ahead and change the DNS setting as I have shown and proceed.

ON XP machine, add a DNS entry to network adapter

Right click "MyNetwork Places" icon
choose Properties

Then right click Localk Area connection
Choose Properties

Highlight TCP/IP adapter and Choose Properties

Uncheck "Obtain DNS server address..." and place check in "Use the following DNS...."
Put the IP address of you Active Directory Domain controller like shown in the example.
DO NOT use my IP address. Change the last number

Click OK.

If you have not already joined your XP computer to you domain controller, then do so.
This should eliminate authentication problems.