what Skurj said from my small exp. in the area...
my Netgear card has it's own software installed, and it has a network chooser.
there are 3 wireless networks in my area, my neighbor lets me use his, and I have the router SN# for the WEP security, and I can't even connect to the others...
sounds like some people around you are running an open network, allowing anyone to connect... if your network card has software I'll bet it allows you to choose the connection like my does automatically.