Now that your product key is changed, you'll need to reactivate Windows XP.
You should now be seeing the Activate Windows by phone screen. This is one method of activating Windows which you're more than welcome to use.
If you click the Back button, you'll see that you have the option of activating over the Internet - a much easier and faster way to activate Windows XP assuming you have an Internet connection on the computer.
If you'd rather postpone activating Windows XP until a later date, you can click the Remind me later button on this window or choose the No, remind me to activate Windows every few days radio button on the main activation screen.