You download Windows Phone Express or whatever stupid name it attached with the Visual Studio Express for Windows Phone, and after a period of some days you get this message, or similar:
Why the F… does Microsoft make me register the software I am using for FREE? Well I won’t go into the lengthy explaination, but click on the button and you will need to sign in using your liveID. You then cut and paste the code into the Registration key textbox.
Now people at Microsoft are happy! Office workers get that data they need for their reports, planning for funds to build the next version, etc. are put into place. Maybe in a few years you might hold a similar job and also be very happy!
Check out the email you get, there are some great links to help you out. I found one that I am going to use.