reply to JohnD
Re: what hardware do I need to do this? First question is whether office A and office B are using the same ISP or not -- you mention "cable modem," but
that doesn't tell us much.
Second jimbopalmer 's suggestion of VNC or RDP, especially if security is not an issue -- I've said before
that VPN is about secure communications over an insecure medium like the internet; nothing else.