There are a number of factors that can cause the signal of your access point to deteriorate and the performance of your network to fall under par. Practically any appliance that operates on the same frequency level (2.4 GHz) as 802.11b or 802.11g can cause interference with your wireless network. Be sure to keep cordless phones, microwaves and other electrical equipment at least 1m away from the access point. Try changing channels on the access point and test it out on one of the clients. To change the radio channel on the access point login to the configuration (usually a web based interface) and go to the Wireless Settings (will vary depending on vendor) section, select a different channel and save settings. On the client, go to Device Manager, right click your wireless network adapter and go to Properties. In the advanced tab select the Channel Property and change the Value to the same number as the one you chose on the Access Point. Disable and then re-enable the wireless connection.