Hi,
Can you get a "good" signal if you place the laptop directly next to the WiFi router or modem?
If so, (or even if not) check that the laptop's two WiFi antennae are securely connected to the WiFi card.
Here is a link to the ifixit [guide|33907] guide.
It shows how to gain access to the WiFi card. In Step.4 the antennae are highlighted.
Hopefully this is of some help.