check the wireless router settings, some routers have a signal strength adjustment. For example, my D LINK router sets up to broadcast in a small local pattern, or can be switched to broadcast 200 foot radius. Your settings may be adjustable.
OR, get a better router with double or triple antennas.
OR, Use a PCI card adapter instead of the USB, with an external antenna. The antenna can be positioned to pick up more signal. Putting the antenna up higher than the computer can improve the signal.
OR, move the computer closer to the router; move the router closer to the computer.
Or, run a hardwire to the computer, a wired connection is faster than a wireless connection.