An aerial on a PCI wireless card tucked away at the back of the computer can lead to poor signal strength as the others have said. I had this problem too. As the aerial was removeable, i bought a desktop aerial and cable from Maplins and the signal strength was restored.
Its not where the card is, Its wire that hangs out of it that matters. If its in a corner its not going to pick much up. Antenna needs to be in free space as far as you can make it, i.e not a corner or back of PC
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