I haven't personally done this, but I would imagine you run pg_dump on the current database and save it to a file, then move that database into your target Ubuntu server and import it. You may need to ensure the postgres versions match when importing, but I'm not sure about that. Nobody can be accessing the database when you do this, so OpenERP needs to be shut down and any tools like pgAdmin can't be running.

export on windows cmd line, navigate to the folder pg_dump.exe is sitting in:

pg_dump -C my_database_name -f my_db.sql

import on ubuntu:

psql -U openerp -d postgres -f my_db.sql

openerp is just the name of the database user in postgres on Ubuntu, that depends on your setup but it's a common one to use. The use of -C prepares the dump to create a new database when executing the file in psql.