The PC that I used as an example had Windows 7 Ultimate installed. I've hidden the product key but you can see that Winkeyfinder found it without a problem.
If you have a Microsoft Office program installed, you can click the MS Office button to display that product key.
If you're using Windows XP, you can change your product key by clicking the Change Key button located under the product key display. If you'd rather not trust a free program to change your product key, you can change your Windows XP product key manually by making a few registry changes.