Can't change my windows product key

[Kurak]

So I want to change my windows product key. I went through the steps to alter the reg key, and I went to change the key using %systemroot%\system32\oobe\msoobe.exe /a

The key won't work though! The old install is windows xp sp2, as is the cd I have here. The key is a 100% legitimate licensed copy.

[djnes]

If you google a program called Magic Jellybean Keyfinder, I believe it shows you the current key, and allows you to change it.

EDIT: http://www.magicaljellybean.com/keyfinder.shtml