read/write for some users, read only form others

I have a database at work which I built in Access 2007, but then had to
save it as a Access 2003 mdb format.
The database was then placed on a NAS server and split. The front end
was then placed on local users PC's, some of which have Access 2003.

I now want to restrict write access to some users and only allow 2 users
to do that, while all can have read access.

How can I achieve this situation and what is the best or easiest way of
doing so.

Thanks