Make sure the modem and the wireless router are turned on.
If they're both turned on and your laptop detects the wireless router, then on the XP computer open Control Panel/Device Manager, double click on Network Adapters, and make sure there's an adapter name that has "Wi-Fi" in it.
If that's ok, then double click on the adapter name, click on the General tab, and make sure it says "This Device Is Working Properly".