Making your client stay home
Fortunately there are two things you can do to keep your wireless clients on their own network. The first is to change your SSID from its default value and one not used by nearby WLANs. Choose something unique and that doesn't divulge your name or location (for security's sake). Using only letters, numbers, underscores and no spaces should give you plenty of options for the SSID name.
TIP: Some APs allow you to set an "AP Name", which has nothing to do with its SSID, but is used to tell multiple APs apart for management purposes. So make sure you change your APs SSID (sometimes called ESSID).
The second step is to clear the Preferred Network list and make sure connection to non-preferred networks is disabled. Some wireless client utilities refer to this list as Profiles, but the concept is the same.
With XP, you'll find the icon for your wireless adapter in the Network Connections window (Start > Settings > Network Connections). Right-clicking on the icon and selecting Properties should bring up a window similar to Figure 9.
Figure 9: Too many "Preferred" networks
The upper portion of the window lists Available, i.e. currently detected, wireless networks while the lower section lists Preferred networks. Simply delete every network except yours by selecting them and clicking the Remove button. Then click the Advanced button to bring up that window, and make yours look like Figure 10.
Figure 10: XP Advanced Wireless Network Properties
This will prevent your card from trying to connect to Ad-Hoc networks (in the unlikely event that any are around), but more importantly prevent automatic connection to any new wireless LANs that appear in your neighborhood.
TIP: If your wireless client is moved to another location where wireless LANs are present, you should repeat the Preferred Network clean-out when you return to your normal location.
For Win 7, the process looks different, but does the same thing. You first bring up the Network and Sharing Center (Figure 11), which you can do from the Network connection tray icon. I've highlighted the Manage wireless networks link that you need to click next.
Figure 11: Windows 7 Network and Sharing Center
The Manage Wireless Networks window lets you set the network connection priority and add and delete wireless networks. Again, make sure that your network name is unique and at the top of this list. You might also want to delete all the other networks that may have made their way into this list. You definitely want to delete any networks that are the same as any in-range SSID's that you can see!
Figure 12: Windows 7 Manage Wireless Networks
To reach other wireless controls in Win 7, you need to get to the Wireless Connection Status window. You can do this by clicking the network connection icon in the tray, right-clicking the currently-connected network and choosing Status to bring up a window like the one in Figure 13.
Figure 13: Windows 7 Wireless Connection Status
Clicking the Wireless Properties button will bring up the window shown in Figure 14.
Figure 14: Windows 7 Wireless Network properties
The Connect automatically... box shown checked in Figure 14 should be checked only for the network you want to connect to. If it's checked on other wireless networks that you want to keep around (Manage Wireless Networks - Figure 12) for when you travel or take your notebook to the office, that fine, as long as that network doesn't have the same name as any in-range networks!
If you're using your wireless adapter's client utility, check to see if has a similar "preferred network" capability, and perform a similar clean-out if possible. Some utilities use "connection profiles", which store all the settings for connecting to a particular WLAN and require you to manually switch among them. You shouldn't have to perform a "clean-out" in this case, but you may need to delete unwanted profiles if your client utility automatically creates them when it detects new networks and automatically switches among them.