If you ever run into a situation where you need to change your product key so you can activate windows, there’s a simple command line technique to do so. Simply open a command prompt and type in “slmgr.vbs –ipk [product key]”
After a few moments, the following message should appear.
When you’re done that, type “slmgr.vbs /ato” to activate windows (see below)
And the following message should appear shortly afterwards if you did everything right.
This works on Windows Vista, Windows 7, and Windows 8.