[Samba] cannot change computer name

[aT]

I have samba PDC configured.

I am unable to change the computer name for any computer . All clients 
are windows XP pro .

It says access denied. Although i use the same Administrator username 
and password which i used to join these computers to samba domain. I 
will really appreciate if some can help me in this .
thanks & regards

I am using samba-server-3.0.13-2mdk

Here is Global section of my samba conf

[global]
        server string = network Share
        workgroup = NETPDC
        netbios name = freeze
        encrypt passwords = Yes
        smb passwd file = /etc/samba/smbpasswd
        passwd chat = *New*password* %n\n *new*password* %n\n *success*
        passwd program = /usr/bin/passwd %u
        passwd chat debug = yes
        unix password sync = Yes
        log level = 0
        username map = /etc/samba/smbusers
        syslog = 0
        printcap name = CUPS
        show add printer wizard = no
        printing = CUPS
        name resolve order = wins bcast hosts
        add user script = /usr/sbin/useradd -m %u
        delete user script = /usr/sbin/userdel -r %u
        add group script = /usr/sbin/groupadd %g
        delete group script = /usr/sbin/groupdel %g
        add user to group script = /usr/sbin/usermod -G %g %u
        add machine script = /usr/sbin/useradd -s /bin/false -d /dev/null %u
        logon script = scripts\login.bat
        #logon home = \\%L\%U
        #logon drive = X:
        domain logons = Yes
        preferred master =  Yes
        wins support = Yes
        winbind separator = \
        idmap uid = 10000-20000
        idmap gid = 10000-20000
#       directory security mask = 0775
#       security mask = 0775
#       create mask = 0775
        directory mode = 0775