How to solve "Silverlight.CSharp.targets was not found" problem

Friend of mine asked that she can't build any Silverlight project inside VS 2010 from http://gallery.expression.microsoft.com.

The error was similar to : 

C:\OverlappingTabs.csproj : error  : Unable to read the project file 'OverlappingTabs.csproj'.
C:\OverlappingTabs.csproj(108,3): The imported project "C:\Program Files\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.CSharp.targets" was not found. Confirm that the path in the declaration is correct, and that the file exists on disk.

This problem runs because VS2010 came up with support for Silverlight 3 but not Silverlight 4 as Silverlight 4 had released after VS2010.

What you need is just setup "Microsoft Silverlight 4 Tools for Visual Studio 2010" about 35 MB without restarting your OS.