[Samba] local admin permissions

[Ninti Systems]

We have implemented an NT domain with samba 3.0.4 PDC on Slackware 9.1
and Win XP Pro clients. Everything is working nicely except for one
issue. I haven't used Windows for years, so possibly I just don't
understand something basic about Windows XP security ...

Having logged in (to the domain), users are unable to perform tasks such
as update certain files on the local system, for example to update
anti-virus software or install a newer web browser or browser plug-in.
Also, some applications run locally (eg MYOB) complain about not being
able to open files they need (probably tmp files).

I have added these users to a Unix "domadm" group and mapped this group
to "Administrators" using 'net groupadd', to no apparent effect. 

How can I configure things so that users on Windows clients can perform
the tasks described? Am I on the right track with 'net groupadd' etc?
Does samba even have anything to do with this? 

In a nutshell, while I understand something about how samba restricts
user access to resources on the samba server, does samba also dictate
the permissions a user has on their local system? The local user
permission settings appear to be being overriden by settings obtained
from the samba server.

Thanks, any ideas greatly appreciated.

Mick