You should also change the default SSID, for gods sake don't leave passwords as dfault eg 'admin'.
Use WEP, definatly use MAC address filtering (it is probably the most secure method).
Oh Lord AC if without any set-up at all your girlfriends laptop will connect then your network is not secure. My advice is to turn it off until you ahve worked out how to secure it.
For those interested in the flip-side: wardriving is the term for it, wardriving is illegal and unfortunately (for those running wireless networks) a growing problem because of the relative ease with which it is done using freely available software.