ooohh I see, well if they don't know that your piggy backing their connection, which is ilegal by the way, then I'm not sure how you can go about improving it. You may improve it by installing a new wireless adapter, some of the newer usb ones claim to improve reception but its a gamble. If they know about you borrowing their connection then you may want to have a look at getting a signal reapter, this may improve it a little, you could also ask them if they could try the router in a different place to see if things improve.
If its an unsecured line and they dont know about you then I dont see that its worth forking out any money to improve it because they may choose, in the future, to add encryption, that would leave you without a connection and out of pocket.