the following things can interfere with WLAN.
-cell phones: some models transmit on the same frequency as the router, and transmit data even while not being used. move it away from the computer or router or turn it off.
-cordless phones: newer models of cordless phones often use the same frequency as the network. try moving the base station to another location or use a lower frequency model.
-microwave ovens: these work at the same frequency of wireless devices. if it is near the router or the pc or between them, it will cause interference while running.
-distance and obstructions: the further the pc is from the router, the more prone the signal is to interference. objects between the router and the pc will weaken the signal. the fewer walls, floors, furniture, etc between the better the signal will be.