We lost our wireless connection less than 50 linear feet from the access point in a wood-frame house. Many wireless users have similar complaints. Wi-Fi, officially known as the 802.11b standard, is supposed to be better than that. But vendors invoke very broad caveats because of the many causes of dead zones in a Wi-Fi network. Many buildings have structural elements that can block radio transmissions, including Wi-Fi. Even passing through wood and drywall may limit the power of your access point's signal. In addition, Wi-Fi uses the same basic radio technology as 2.4-GHz cordless phones. Such phones, and especially their base stations, can cause interference. If you have a phone base and a wireless AP, keep them as far apart as you can.
Microwave ovens are another major source of Wi-Fi interference. These devices throw out enough radio waves in the Wi-Fi range to make communications unreliable within a few feet of a microwave. But interference occurs only when the microwave is in use, and it usually extends no farther than 10 feet from the oven.
Source: www.bcoms.net