Did you try to run Visual Studio 2010 Express for Windows Phone and were promoted to register it?
Like this:
Did you think Visual Studio 2010 Express for Windows Phone was free?
Well, it is. Just register. We don’t ask much. And you it’s a fast response.
Like this:
Thank you for registering your copy of Microsoft® Visual Studio® 2010 Express for Windows Phone. Below you will find your product activation key and some additional resources to help you get started creating apps for Windows Phone. Unlike other Express products, Visual Studio 2010 Express for Windows Phone integrates with Visual Studio 2010 Professional and higher.
Now, start developing for Windows Phone.
Best of luck!