Yes, by default it is set up to be secure.
There are four steps to make sure your database is accessible from outside your internal network.
(1) Edit postgresql.conf (Start --> Programs --> PostgreSQL --> Edit postgresql.conf), set listen_addresses = '*'.
(2) Edit pg_hba.conf (Start --> Programs --> PostgreSQL --> Edit pg_hba.conf) and add a line at the bottom to allow the IP addresses you want in. If you want to allow all IPs to connect, 0.0.0.0/0 will allow everyone in. Make sure it says md5 on the line, not trust. Basically copy the line that is there by default for 127.0.0.1 (if you edited that to be trust, in the new line make sure it's md5). If you know you're going to be in a specific place, you can restrict this more as you please; it uses
CIDR notation.)
(3) Restart PostgreSQL (Start --> Programs --> PostgreSQL --> Stop Services, followed by Start --> Programs --> PostgreSQL --> Start Services).
(4) Set your router to forward port 5432 to your PostgreSQL server.
Then you will be able to connect to your PostgreSQL server at your IP address from anywhere else on the internet. (You will need to know your external IP for your computer, of course.)