What kind of signal strength are you getting? You can try moving your computer around so that the antenna may get a better signal. Also, if the access point has adjustable antenne, try moving them around.
It's best if you use the signal meter on your pc while you are doing this (usually installed when you installed the USB thingy).
Also, make sure there are no 2.4GHz telephones around, as this can cause this kind of problem too!