Here's a question which I receive regularly:
Question: How do I change my Windows product key?
How you do this depends on the version of Windows you are running. If you are running Windows XP then the easiest way is to download the Windows Product Key Update Tool - this is a legitimate Microsoft tool so you can use it with confidence.
If you are running Windows Vista then you can change the product key directly from within Windows (Click Start, right-click on Computer and select Properties and then click Change product key that appears at the bottom of the box, then follow the prompts).
Remember, changing your product key will mean that you will need to reactivate Windows after changing the product key.