Metageek’s inSSIDer is an Open-Source Wi-Fi scanner, a simple yet powerful tool for discovering the wireless networks around you, including security information, the strength of the network compared to where you are, and what channel each network is broadcasting on. inSSIDer uses the wireless card that comes with your laptop or computer – or maybe an external USB Wi-Fi adapter if you don’t have one built in. But – inSSIDer is 100% dependent on the hardware you have in your computer. inSSIDer is the best free Wi-Fi network scanner for Windows 7, Vista and XP. Because NetStumbler doesn’t work well with Vista and 64-bit XP, MetaGeek built an open-source Wi-Fi network scanner designed to work on all current generations of the Windows operating system. inSSIDer has been reviewed by Lifehacker and Tekzilla and has been downloaded over 1.5 million times.
Download inSSIDer here: http://www.metageek.net/products/inssider/download/
Download the inSSIDer User Guide here: http://files.metageek.net/marketing/MetaGeek_inSSIDerUserGuide_WiFi-Scanner_2012.pdf
inSSIDer is a really useful starting point and I highly recommend it as a first step. If you want to really see what’s going on though, MetaGeek makes a range of USB devices called Wi-Spy as well as companion software called Chanalyzer, that visualises everything that’s going on in your wireless network. Wi-Fi interference can’t be detected by inSSIDer, but when you connect a Wi-Spy to Chanalyzer – you can see all the interference around you.