samba - Jan 2012 - problem adding admin to widows 7-computer

Hi,
I have a brand new HP laptop with Windows 7 which is supposed to be used within
the SAMBA (3.5.4) network. I can add the computer to the network without problem
and login through it with normal SAMBA-users.
?
I would however like to add an administrator to the particular computer in
"samba-mode". I have been able to do this through the computer twice,
but after a couple of restarts the administrator is lost. How can this be?
/Anna-Karin