PT2 or PT3?
PT3 does not yet have built in database backup, so you will have to use PGAdmin (part of postgres) or a DOS prompt (see
Database / Postgres FAQ for a link) to back up your database.
Alternatively you can use either the 'processed' hand histories if you kept the original files, or a hand history 'Export' (Database -> Database Management) to import into a new database on your new computer.