Project cannot be opened because its project type (.csproj) is not supported by this version of the application.

To open this project, please use a version that supports this type of project.

Follow the below steps to resolve the above error.
Click Start> All Programs > Visual Studio 2008 > Visual Studio Tools > Click Visual Studio 2008 Command Prompt. Type the below command and press enter.

"devenv.exe /resetskippkgs"

Executing this command should solve your problem.