A couple of quick pointers, be sure that your using the most recent drivers for network card and if possible check the firmware in the router are current (sometimes beta are better so maybee read forums for the models you have).
Most likely casue is that the signal strength is too weak, this is usually cause by the router being too far away or seperated by thick solid walls which dont allow wireless signals to pass through clearly, most often people find that they are using the same channel that is being used by neighbours, usually the software that comes with the card shows other networks besides your own and what channel they are on, basically you want one without any others on it, if you cant find this in your software download a program called netstumbler it will scan the frequency and show up any broadcasting networks and what channels they are on.
good luck
BS
