Visual C++ :: Cannot Find Relative Header File

For a while, I have been running a Windows 7 Visual Studio 2012 CPP project fine. However, today, all of a sudden, it has stopped being able to read any header files that are in relative directories. I cannot think what I have done to make this happen.

It seems to be an issue with Visual Studio as a whole, rather than my individual project, because I have created a new simple project and I am having the same problem. So, I have a CPP project at `C:/Projects/TestProject`. Then, I have a header file called `test_header.hpp`, which is located at `C:/Headers`. In `TestProject`'s project settings, I have added the directory `../../Headers` to `Additional Include Directories`. Then, my main functions is as follows:

Code:
#include "test_header.hpp"
int main() {
return 0;
}

Which gives me the error:

Code:
Error1error C1083: Cannot open include file: 'test.hpp': No such file or directoryc:projects estprojectmain.cpp11TestProject

Is there some global Visual Studio setting that is responsible for setting up these relative directories? Just in case the relative directory was not correct, I also tried a load of others, such as `../Headers` and `../../../Headers`, but these gave the same errors.