Sharing Internet connection.

Our company has decided that it’s time to change our ISDN connection to ADSL. What’s the best way of having a secure connection?

Would it be possible to have anti-virus installed at each machine and using the router’s firewall? Is the router’s firewall going to offer suffcient protection?

We are a small company with 5 sites and typically 10 machines at each site.

Can anybody help?