inSSIDer is a free Wi-Fi network scanner for Windows Vista and Windows XP. Because NetStumbler doesn’t work well with Vista and 64-bit XP, someone built an open-source Wi-Fi network scanner designed for the current generation of Windows operating systems. inSSIDer was discussed by Lifehacker and Tekzilla

I’ve not tried this myself, but got it reccomended, it indeed looks cool and worth a look..

get it here;