I've run into this a few times dealing with XP machines and workgroups.
By default if your XP system is in a workgroup you will need to change the following setting otherwise you can only authenticate as a guest.
Start --> Run
Secpol.msc
Local Policies --> Security Options
Change:
Network Access: Sharing and security model for local accounts
to Classic - Users authenticate as themselves.
Give this a try.
Jay