If you want to open a project (.csproj) file that was created in a previous version of Visual Studio, then you need to run Visual Studio (devenv.exe) from the Visual Studio Command Prompt after providing options to clear all SkipLoading tags.
So you can go to Start Menu -> All Programs -> Microsoft Visual Studio 2010 -> Visual Studio Tools -> Visual Studio Command Prompt. Once you are in the command prompt, enter:
Devenv.exe /ResetSkipPkgs