http://download.cnet.com/inSSIDer/3000-18508_4-10848357.html

this program shows the signal quality(dBm) and if your w-lan is interfering with other w-lans near by.
if you have a movable antenna, you can put it at the best place by trying and watching how the dBm changes.

got mine from -55dBm to -41dBm by just putting the antenna a few centimetres to a different place.