How can I create a copy of postgis db from one windows machine to another
As @a_horse_with_no_name wrote pg_dump and pg_restore cope with PostGIS things without problems. At least since PostGIS 2.0, previously (before PostGIS became an extension) you might had to change paths of the c-library used for geometry-functions.

In PostgreSQL since 9.x ist is also fairly easy:


backup at the original databse with pg_dump
at the new system: 


create a new database
login to that new database and run CREATE EXTENSION postgis;
restore the backuped file with pg_restore to the new database



That works for me as well on windows as on linux, you could even use PgAdmin for it.

Note: the version of pg_dump of the original (backed up) system may not be newer than of the new system - see https://stackoverflow.com/q/12836312/8957103