If you are using WPA or WPA2 security (you should), but you've failed to successfully connect, then one side may blacklist the other and connections are prevented.
To rectify this and try again, unplug power to the router and reboot the XP machine. Restore power to the router. If they continue to lock one another out, then go into the wireless network settings on the XP machine and erase any saved wireless profile information you've saved (it may be incorrect or corrupt). Reboot both again.