try this 1
If you are having problem only with your wireless then download the wireless drivers.
Go to your manufacture website, just click on this link
select the following options
product list = notebook computer
series = satellite
model = L640
Select your OS type
and in category ---> download Ethernet LAN and Wireless LAN drivers by selecting the options and clicking on Show Drivers ---> then install it.
Note : select the right drivers for your laptop.