If you are using WLAN, this piece of software is for you: inSSIDer. It shows you the signal strengths and channels of the wireless networks visible to your computer. With inSSIDer it is very easy to see competing networks, i.e. networks that are on the same channel and thus interfere with each other. Many people just leave their network to operate on the default channel which also contributes to the risk of using the same channel as your neighbor. inSSIDer also draws a graph about the signal strengths as functions of time so you can, for example, walk around your apartment with your laptop and determine if your WLAN reaches every corner of the place equally well or if you should move your access point to ensure better reception. I have also tested that inSSIDer works on Windows XP and on Windows 7.
Oh, and one more thing. What is this "SSID" that's highlighted in the name of the program; why not just "Insider"? Wikipedia explains: SSID stands for service set identifier, which is just a fancy name for "the name of a wireless network".