DO the user credentials on the windows boxes match the credentials on the samba box? Did you turn off simple filesharing on the XP machines?
in an effort to prove that at least my creds match, I dropped from root to my regular user status in shell and did smbpasswd
this is what happened:
Old SMB password: New SMB password: Retype new SMB password: Could not connect to machine 127.0.0.1: NT_STATUS_LOGON_FAILURE Failed to change password for ray