Silverlight project Error Cannot get the list of output files from Project

This problem appear to web because I opened some forms with
Expression Web 4.0 and it seems to have modified my project files.

I finally found that to solve it all I have to do is to:

1. Open the .csproj with a text editor
2. Look for the ToolsVersion an change it from 4.0 to 3.5

And that’s all.