Sounds like quite a challenge. 50 PCs on a net can be a difficult size to manage--too small for expensive administrative tools, too big to treat administration as a part-time job.

Sounds like your branch offices are tied to HQ via private lines, to form a closed network. If your server is acting as DNS, then I'd go with what SDG offerred and let it assign IP addresses to each PC using DHCP. The DNS should be configured to do that automatically, and you just need to ensure that all PCs are configured to get the "address from the network", Microsoft's dumbed-down way of saying DHCP.

Your PCs should point to the company DNS server, and their gateway address should be your firewall.

I was looking at the GeekSpeak TOC, and it seems like you really have a challenge in front of you, with this plus backup and Windows Update to worry about. Keep on posting here and we'll see what we can do to support you.