Try netstumbler
NetStumbler is a tool for Windows that allows you to detect Wireless
Local Area Networks (WLANs) using 802.11b, 802.11a and 802.11g.
It has many uses:
• Verify that your network is set up the way you intended.
• Find locations with poor coverage in your WLAN.
• Detect other networks that may be causing interference on
your network.
• Detect unauthorized "rogue" access points in your workplace.
• Help aim directional antennas for long-haul WLAN links.
• Use it recreationally for WarDriving.[sub][/sub]
Download from
www.netstumbler.com