Visual Studio 2005: Project file can not be opened

I have a big solution file and few projects can not be loaded. I do have all the files in the proper directories. But when I right click and try to load the project, I get the following error:

The project file "C:\myapp.proj" can not be opened. The project type is not supported by this installation.

By the way, these projects are related to Windows Workflow.

Please help.

I am using Visual Studio 2005 on Win7 machine.

It might be a modeling project, test project, silverlight/wpf project etc. It's definetely something that's not supported by your version of Visual Studio, you should use a newer version or a different one (i.e. professional instead of express).

Update: I did not see that it was Windows Workflow Foundation. Check out this link, it's a toolkit for using Workflows in VS2005.