You are going to get conflicting answers on this. For example Microsoft "blurb" says...
http://www.visualstudio.com/en-US/products/visual-studio-express-vs[
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Quote:
Commercial use of Visual Studio Express
Visual Studio Express products are available at no charge and may be used for commercial, production usage subject to the license terms provided with each product. For example, you can use Express for Windows to create apps that you can then submit for sale in the Windows Store.
But some of the earlier versions specifically said it couldn't be used for commercial purposes.
Try downloading it and actually reading the Terms & Conditions before clicking OK ;-p
or have a search through this link
http://www.microsoft.com/en-us/legal/intellectualproperty/UseTerms/default.aspx[
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There is also a BizSpark program which you could try -
http://www.microsoft.com/bizspark/default.aspx[
^]. If you try this route make sure you put forward a reasonable business case and ensure that email addresses are appropriately named to imply you mean business (excuse the pun)