If this is a sudden issue you may have a issue with the antenna connections to the internal wireless unit. You could disassemble the machine and carefully remove and reseat the cables going to the antennas. You could also if you are going to all the trouble replace the internal card with a N based card.
The simplest thing to try is a $10 usb N wireless adapter. N tends to get better coverage because it uses a more effective encoding method. Your router would also have to support N.
In some cases you can't do much if the distance is far or you have a lot of walls absorbing the signal.