The antenna on most laptops is in the lid which means you have to remove the entire lid, hinge assembly and screen bezel. The wires from the antenna then run down via the hinges (As does the screen connections) and along channels to the Wireless card. A lot of hassle and the cost of doing all this would be more than buying a wireless USB adaptor as Woolwell suggests.
These days you can get very small one's such as this Edimax WiFi dongle for less than a tenner. It will stay in the computer pretty unobtrusively and the only downside is that it will occupy a USB slot almost permanently.
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