I installed MVC4 side-by-side with MVC3.
When I create a new MVC4 web app project in VS 2010, I get this error:

There is nothing about that on Google.
Any idea ?
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Thank you to AndyM for the help.
Here’s how I solved the issue:
1. Uninstall NuGet in Windows Add/Remove Programs
2. Opening the following folder
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\Extensions\Microsoft Corporation\NuGet Package Manager3. Delete all folders and files there
4. Uninstall MVC 4 in Windows Add/Remove Programs then reinstall MVC 4