What you want is called "virtual private networking."

Both computer networks connect to the Internet in whatever manner they choose.

Then one of them has to have a "VPN server" in the network. All computers in both networks have to connect to that VPN server, who will serve up crypto keys to the clients.

The clients create a "tunneled" connection, where all your traffic between the two networks is encrypted, so your file contents, etc, can't be seen by someone sniffing packets on the Internet.

Do a google for "Windows VPN" and start reading up on VPN's. Windows (since Win2K) has a VPN client-side tool bundled. You'll need a VPN server on one or the other networks to connect to.

Many higher-end DSL routers also have VPN services built into them, if you'd rather offload the crypto issues onto non-Windows hardware.

The good news is that what you want to do is a) common and b) widely supported and c) pretty well shaken out - -ie, it isn't new.

The bad news is that there are about a half-dozen ways to do what you want to do, all with their own good and bad points, so you're going to have to bone up on VPN's and the products offered to do it and come to a decision as to how you want to run your VPN going forward.