Use a shared folders and a network cable if they have gigabit network ports, otherwise copy files via an external hard disk.
(if they both don't have gigabit network ports they probably have fast ethernet which is 100mbps, it will just take longer to transfer files.)
You can use Firewire, Lan cable 100/1000 mbit or WiFiG if you have a wireless modem/router in your house. There are other ways too but these are the first I can think of.
