Here i told you a method how to change it,
1.Download keyfinder,download here.
2,Download window xp sp2 key.download here. (the product key will lost after 80 day in store server)
3.Unzip the keyfinder and run keyfinder .exe,you will get as below screen shot;
4. Copy down your computer key that showed in the CD key column,then click ‘option’ and select ‘change window key’.
5.Copy the key that downloaded and paste it,
6.Then click ‘file’ and select ‘save’ to save it.
7.Now your computer is with new product key,it is need to have validation.
8.Use firefox web browser (remember,don’t use IE),go to validation link,
English version or Chinese version.Depend the language in window xp.
9.Click the ‘validation window’, you need to download WGA plugin and install the WGA plugin.
10.After install plugin in,go back and click the ‘validation window’ again.
11.If validation successful,it will showed ‘Thank you for using the Windows Genuine Advantage program. You may now access resources for genuine Windows users”.you can update your window through Microsoft.
12.If unsuccessful,repeat the step 5 to 9 with different product key till validation successful.
Good luck to enjoy.
p/s :Maybe some of the the product key that provided already used by other,then you need change it.