Enabling Code First Entity Framework VS Web Express 2012

I am trying to enable Code First in my Visual Studio Web Express 2012. From what I have read so far I have to use the Package Manager Console and the command Enable-Migrations. When I do this I get the following error.

Exception calling "LoadFrom" with "1" argument(s): "Could not load file or assembly 'file:///E:\Working\wingtoysdl\C#\packages\EntityFramework.5
.0.0\tools\EntityFramework.PowerShell.Utility.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515)"
At E:\Working\wingtoysdl\C#\packages\EntityFramework.5.0.0\tools\EntityFramework.psm1:398 char:62
+     $utilityAssembly = [System.Reflection.Assembly]::LoadFrom <<<< ((Join-Path $toolsPath EntityFramework.PowerShell.Utility.dll))
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

You cannot call a method on a null-valued expression.
At E:\Working\wingtoysdl\C#\packages\EntityFramework.5.0.0\tools\EntityFramework.psm1:399 char:50
+     $dispatcher = $utilityAssembly.CreateInstance <<<< (
    + CategoryInfo          : InvalidOperation: (CreateInstance:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

Exception calling "CreateInstanceFrom" with "8" argument(s): "Could not load file or assembly 'file:///E:\Working\wingtoysdl\C#\packages\EntityF
ramework.5.0.0\tools\EntityFramework.PowerShell.dll' or one of its dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515
)"
At E:\Working\wingtoysdl\C#\packages\EntityFramework.5.0.0\tools\EntityFramework.psm1:431 char:31
+     $domain.CreateInstanceFrom <<<< (
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

As you can see it is a very large error making it very hard to google and troubleshoot. Does anyone have any ideas on how I can fix this?

I've run into this issue twice now. Both times I've had to uninstall entity framework then re-install it for each project in the solution. One thing that took me a while to figure out the second time I did this is that I had to re-start Visual Studio (2012) after re-installing Entity Framework (or I continued to get the "Could not load file or assembly '[path]\packages\EntityFramework.5 .0.0\tools\EntityFramework.PowerShell.Utility.dll'" error).

To get to the Package Manager Console go to: Tools -> Library Package Manager -> Package Manager Console

In the Package Manager uninstall Entity Framework for each project (I selected each project from the Default Project dropdown at the top of the Package Manager Console):
Uninstall-Package EntityFramework -Force

Then install it for each project:
Install-Package EntityFramework

At this point attempting to run the Enable-Migrations command gave the error still, until I closed and restarted Visual Studio. Then it was successful, as was the Update-Database command (after setting AutomaticMigrationsEnabled to true in the Migrations.Configuration file).