You can try another RF channel to see if you can make your own network, but your discovery is not rare.
Many laptops have a Wifi "off" button for such occurances.
Bob
I recently got a 802.11g card for my laptop to use when I am traveling. I plugged it into my computer and it detected to network and I was able to connect to it. I was surprised since I did not purchase a router: so I'm mooching off someone else's network.
I live in an apartment in a densely populated area so I guess I shouldn't have been surprised. When looking at the connection properties it says the connection is excellent and running at 11Mbps; so whoever has the network is using .11b. Also under the properties it say the network name is "default".
I already use Zone Alarm for my firewall but was wondering what I should do? I have no idea whose network this is and now stable it will be. Is there a way to secure my computer further so the "owner" of the network can't see my information? Is there a way, besides going door to door, to find out whose network this is?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic