inSSIDer

Inssider is a tool designed and developed to replace Network Stumbler due to the lack of support for Vista and due to the long time between updates
Its first and only available version was released on March 2008; with a basic network discovery, monitoring and graphing feature, I found that it actually works quite well for a first release, something that speaks well for Inssider developers Charles Putney and Norman Rasmussen.

Like most wireless network discovery and monitoring tools, Inssider works with internal Wi-Fi radio, it display the network information (SSID, MAC, data rate, signal strength, security, etc) in a graph signal strength over time. It doesnt implement GPS support yet but I expect it in future versions in order to compete with Vista Stumbler, another tool that was created due to Network Stumbler issues mentioned above.
Inssider is a free Open source tool under Apache License, Version 2.0, I believe it can catch Vista Stumbler features and maybe match Network Stumbler capacity if its developers decides to work really hard on it, for the moment I would suggest you to use Vista Stumbler or stick with XP.