log into your wireless router. change the wireless security type from wpa2 to wep-open. also change the wireless network name. be advised all of your wireless devices will be disconnected from the network when you do this.
now try to connect with your windows xp computer. if it works now on the new network then go ahead and connect the other wireless devices to your new network. if it still doesnt work you can change the router's settings to what they were before.