Question : join a computer to the domain

I have created a user as domain user in AD, I delegated permission at the domain level  so that he can create computer objects in the domain.
in local policy/user rights at the domain level I added him to join workstation to the domain.
in Windows XP, I made the user a member of local administrator.
Now when the user tries to join the WXP to the domain he gets an error message:
Your computer could not be joined to the domain because the following error has occured: Access Denied.

Can someone help through this?

Thanks

Answer : join a computer to the domain

I guess I figured the way to delegate a domain user to join the computer to the domain while this computer object has been already created in a specific OU.
-Delegate control at the OU level where the computer object is created
-select create a custom task to delegate
select only the following objects in the folder
-select computer objects
-select both check boxes "Create/Delete selected objects in this folder"
-I selected Full control ( here I am not sure if it's too much privileges)

Is that safe guys???