Suddenly missing System.Web.Mvc in my project and can't compile

I am using Visual Studio Premium 2013. I have been working on several different C# projects, and joined them all into one main solution. The main project is a C# Razor MVC project. One of the projects was configured for .Net 4.5, but the others were configured for .Net 4.0. So I raised them all to .Net 4.5, After that was done I ran the command: "Update-Package" in the Nuget Console, updating all references.

All looked nice, but I hadn't restarted Visual Studio yet. After I did that I couldn't compile because I was according to Visual Studio missing System.Web.Helpers and near everything that was referenced into the different projects. So I manually added many of the missing references as extensions in the projects, and I ran the following command in the Nuget console:
"Install-Package microsoft-web-helpers." I was now rid of the System.Web.Helpers error, and intellisense wasn't complaining anymore. When I then tried to compile the whole thing, i got(translated from norwegian);

Serverfault in the program.
Compilation error.

Description: An error occurred during the compilation of a resource required to service this request should work. Please review the specific error details below and modify your configuration file.

Compiler Error Message: CS0234: The type or namespace name Ajax does not exist in the namespace System.Web.Mvc (Maybe you lack a collection reference?)

Source error:


Line 29:         <add namespace="System.Web.Helpers" />
Line 30:         <add namespace="System.Web.Mvc" />
Line 31: 		<add namespace="System.Web.Mvc.Ajax" />   <---- This one.
Line 32:         <add namespace="System.Web.Mvc.Html" />
Line 33:         <add namespace="System.Web.Routing" />

Source file: c:\Users\name\Documents\stash_prosjects\myProject\Web.config    Line: 31 

Missing System.Web.Mvc.Ajax

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The System.Web.Mvc namespace is added as a reference, I have verified it, and it also always has been. I guess there has happened some configuration error when I changed the .Net version configuration to 4.5? What to do? Set everything back to 4.0? I notice that I can't navigate to ANY of my usings anymore with F12. Pressing F12 on System.Web.Mvc.Ajax gives me the error: cannot navigate to Ajax. The same is for using MySql.Data.MySqlClient; cannot navigate to MySqlClient.