The project type is not supported by this installation

Whenever I try to open a project (csproj) that's downloaded from the internet, most of the times, I get the

"The project type is not supported by this installation"

It appears that my Visual Studio installation is not corrupted (I can pretty much do everything with it other than open these csproj files)

What may be causing this?

Open up the .csproj file for your solution in wordpad or some text editor. Look for the ProjectTypeGuids. They indicate the required supported types for your solutions. Search the internet these GUIDs to find out what they require. For example E53F8FEA-EAE0-44A6-8774-FFD645390401 means it requires "MVC 3.0"

edit please see the answer further down, which is about 18 months newer, and actually solves the problem. This historically once-accurate answer is no longer as accurate. Leaving intact after the break for this reason. - thanks - jcolebrand

What edition of VS do you use? VS2008 Express, Standard, Pro or Team System? VS2010 Professional, Premium or Ultimate? I would expect that the project you downloaded was created using a higher edition of Visual Studio and uses some of those advanced features. Thus you can not open it.

EDIT: It is also possible that you lack some advanced frameworks like newer versions of Windows Mobile SDK, but if I recall correctly,the error message in such case is different.

For Visual Studio 2010 (prolly also for other versions):

If you are opening an ASP.NET MVC project make sure that the correct MVC version is installed on your PC. If you try to open an ASP.NET MVC 3 project, first close all your visual studio instances and install MVC3: http://www.microsoft.com/en-us/download/details.aspx?id=1491

For other ASP.NET MVC versions download them from www.asp.net/mvc or via Web Platform Installer 4.0.