[Samba] Newbie question for samba 3.0.28 configuration

[Lunix1618]

Hello everyone,

I am new to samba so after some trial, I got stuck with my setup, so I 
decide to post here to look for your assist, so thank you first! and 
below is my installation requirement:

I have 01 server ( running CentOS 5.2) and I will use it as file server 
for 20 workstations that running Windows XP. The setup look easy at 
requested that:
- 01 Directory will permit EVERYBODY in office access with read/write 
permission, I name it PUBLIC
- 01 Directory only permit some persons in office access with 
read/write, another will not permited. I name it ADMINISTRATIVE

I can make it work with the PUBLIC directory but how can I configure it 
to limit access on ADMINISTRATIVE directory

I tried "security = user" and "guest ok = yes" in [global] section of 
configuration file and put "guest ok = no" in section [ADMINISTRATIVE] 
but when I put an user name I not permit to write on ADMINISTRATIVE 
directory when asked, it still can write to it.
So what am I wrong and which permission I need to set on that directory ?
I've used Windows XP to test access to it
My samba version is 3.0.28 (which shipped by CentOS 5.2)

Thank you for your assist!
regards.