Default user account in XP Mode is XPMUser. When adding other user accounts, if you choose account type as Computer Administrator
you can use this account to log in.
If you want to create a Limited user account
with no administrative rights, you have to add it to a group called Remote Desktop Users
before being able to log in to XP Mode. Limited users not belonging to this group can't log in, instead they get an error dialog telling they have no permission every time they try to do this: