Some XP machines do not connect to WPA2 networks. To make this hard, most folk can't find drivers so there is little to do other than replace the wifi with some other card or usb stick.
I have an HP laptop running Windows XP almost five years old which contains an Intel Pro Wireless 3945ABG network device. It connects fine with my router WiFi running off DSL. I recently bought an iPhone 4 and set it up as a personal hotspot. I've been able to connect a friend's Macbook to it. But even though my HP laptop sees the iPhone network, when I try to connect to it I get "Unable to connect to this network. It may be out of range." I've downloaded the current update for the wireless device. Is this a known problem? Is there anything else I should try?