anytime you have a user permissions problem with samba you need to check three things atumatically:
1) The username listed in the smb.conf file IS a legit user account on the linux box
2) The username listed in the smb.conf file has been added to the samba base with smbpasswd -a
3) The username listed in the smb.conf file IS logged to the windows xp pro box and the password matches whats in smbpasswd for that user.
Windows xp pro will allow you to login to a network share as another user but only if one of the following is true:
the xp pro box is part of a windows 2000/2003 or better Active Directory domain and the other user account can be validated on the domain controller
or
the xp pro box has been setup with multiple users and you are logging into the network with the user name and password of one of the OTHER predefined users on the system.
Iv'e not been able to get windows to cooperate in other conditions than these.