Your Computer is Too Far From the Wireless Router or Access Point
If Windows does not detect a network that you think is in range of your computer, it could be because your computer is too far from the wireless router or access point.
With 802.11b or 802.11g routers and access points, the maximum range is up to 150 feet (46 meters) indoors and 300 feet (92 meters) outdoors. With 802.11a routers and access points, the maximum range is 50 feet (15 meters) indoors and 100 feet (30 meters) outdoors. These ranges are in optimal conditions with no interference. Make sure that your computer is within this range and as close as possible to the router or access point. If the computer is portable, try moving it around to determine the range of the wireless signal and the best place to put the computer.
If you are unable to get closer to the router or access point, you might want to consider buying and installing an external antenna to your wireless network adapter. Many wireless network adapters are set up so that you can attach an external antenna to them, which will provide you with better reception than the built-in antenna. Check the information that came with your wireless network adapter to see if you can install an additional antenna.
Note: This information provided by Microsoft.