I\'m in the process of deciding whether or not to use Visual Studio 2012 Express for Desktop or purchase a retail copy of Visual Studio 2012 Professional for my desktop prog
As you receive a product key if you register I think it confirms that you then are free to use VS 2012 Express as you like.
http://www.microsoft.com/visualstudio/eng/downloads#d-2010-express say:
"After installation, you can try this product for up to 30 days. You must register to obtain a free product key for ongoing use after 30 days."