WLAN USB NOT CONNECTING TO WLAN ROUTER

We have a DSL connection that is being ran in to an SMC barricade wireless 2.4 Ghz router that is a lan connected pc up stairs next to the dsl modem and a pc downstairs that is connected to the wirless router via wireless 2.4 Ghz usb adapter. Well the wirless connection will stay up for an A day or two then it will dissapear. we have tried resinsatllation, calling SMC and even updating the drivers files which WIN XP wont recongnize.( pc upstairs WIN 98 SE & pc downstairs WIN XP). DOES ANYONEELSE HAVE THIS PROBLEM? If so how do you correct it? Is there any interfernece from other appliences?PLEASE HELP