Hello,
to activate Windows XP, go to Start > All Programs > Activate Windows. Once there, you should be prompted with the option to activate over the Internet or over the phone. I believe the online activation servers for XP have been shut down a long time ago, but the phone option does still work. Select the country you live in and call the number it provides. At the bottom of the activation window is a long number broken up into 9 pieces. The automated activation line will ask you for those numbers. After you give the line the numbers, it will give you back another set of numbers. Type each set of those numbers into the boxes labeled A, B, C, and so on. If it succeeds, you have successfully activated Windows, but if it fails, your product key is not genuine, and you will need to use a different product key. These can still occasionally be found on Amazon or eBay.
hope this helps!