When your computer looks for available networks, it only shows wireless networks that are broadcasting their SSID. Most routers allow the SSID broadcast to be disabled, so there may be wireless networks available to you that you know nothing about. If you want to search for *all* available networks where you are, try using NetStumbler. It can find any available network, even those that are not broadcast.

Also, if you are experiencing interference on your wireless network (either from other networks, or devices like phones or microwaves), NetStumbler can help detect these sources so that you can take steps to eliminate them and improve your wireless connection.