新手請教again (at work, no Chinese typing sorry)

On initial boot, the Atheros 802.11 a/b/g wireless card comes with the Acer laptop detected one public network and over 10 of my neighbors wireless networks (most had security enabled). When I can not connect to the public one, I tried to connect to those of my neighbors (with no security enabled):))), but I can connect none. Just want to know why and if purchasing an extra adapter card will help to pick up the signal? Thanks for any information.