Something happened to my Visual Studio 2008 install, and I was no longer able to compile C++ projects in x64 configuration - the build would always skip it. The problem also manifested itself by showing an error message: The operation could not be completed. Unspecified error when I tried to view the C++ project properties in x64 configuration. I ran a repair installation of Visual Studio and Service Pack 1, it didn’t help.
After many attempts to find the cause, I came up empty. I finally got a suggestion on twitter to try and install Windows 7 SDK. It requires SP1 to be already installed, and installing it solved the problem! I was finally able to build and view properties of C++ projects in x64 configuration again!