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There is two ways to enable/disable your wifi card on most laptops. One is a button or switch on the front or left side of the laptop. As an alternatively most laptops also have an FN key + Fkey combination such as FN+F2 on most Dell laptops.
This will turn on the card. You may still have your wifi connection disabled in windows so you may still have to go into your Network connections (Start -> Settings -> Control Panel -> Network Connections), right click on the wifi connection and enable.