If one Client can do business with more than one Company and one Company
can do business with more than one Client, you have a many to many
relationship. Full stop.
In designing a relational database it's very important to start by
getting the structure right. If you don't, you risk major complications
in future as the users or owners of the database ask for new
capabilities (as they almost always do).
The best way to approach database design is to think in terms of the
various real-world "entities" you are modelling and the relationships
between them. Usually you'll end up with one table per entity plus maybe
a number of tables for ancillary information. You seem to have at least
the following entities:
Company
Client
The Company-Client relationship (embodying the M:M
relationship betweeen Companies and Clients).
Branch office (with, presumably, a 1:M relationship
between Companies and BranchOffices)
Divisions (are these related directly to Companies,
or indirectly via BranchOffices?)
Contacts (does a Contact belong to a Company,
BranchOffice, Division or a Client?)
And that's not the end of the questions. For example,
*Is it possible for a Company also to be the Client of another company?
If so, could it be relevant? If the answer is yes, then a Client is just
a Company that has a Company-Client relationship with another Company.
You'd lose the Clients table and have the CompaniesClients table doubly
related to Companies.
*Likewise, if a Contact can belong to either a Company or a Client, this
is another reason to consider Clients as just Companies.
Once the structure is right, we can think about importing the data.
I've cross-posted this message to the Database Design newsgroup
(microsoft.public.access.tablesdbdesign) which is a better place for
this sort of discussion. You may also like to look up this article on
normalisation:
http://support.microsoft.com/kb/283878/EN-US/