If you’re receiving a weak signal at a given location, you can try to boost the signal on the transmitting side or you can use a more powerful receiving antenna. This latter approach is only available with 2.4 GHz networks, but it can be a quick and relatively easy fix.
To this end, you want to look at adding a high-gain antenna to the existing wireless adapter connected to your PC. This only works with external adapters; if you’re using a notebook PC with a built-in wireless adapter, you don’t have this option. But if your adapter lets you replace its existing antenna with an external one, this is good route to go. Look for something like Hawking Technology’s Hi-Gain Antennas, which you can buy for $60 or so.