The project type is not supported by this installation.

Ever get the message:

One or more projects in the solution were not loaded correctly.
Please see the Output Window for details.

and in the Output Window

The project type is not supported by this installation.

Usually the Output Window doesn't provide any clues either.  The best way to find out is to open the .csproj or .vbproj file in Notepad.  Inspect the <ProjectTypeGuids>. 

For example,

<ProjectTypeGuids>{E3E379DF-F4C6-4180-9B81-6769533ABE47};{349c5851-65df-11da-9384-00065b846f21};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}</ProjectTypeGuids>

These Project Type GUIDs tell Visual Studio what types of project(s) the file contains.  You can Google the "guid" to find out what it pertains to if you cannot find it here:http://www.mztools.com/articles/2008/mz2008017.aspx

Here are a few other popular GUIDs:

{E3E379DF-F4C6-4180-9B81-6769533ABE47}: Asp.Net MVC 4
{fae04ec0-301f-11d3-bf4b-00c04f79efbc}: C# project
{349c5851-65df-11da-9384-00065b846f21}: Web Application