C++ code reference - Find process window

Code:
HWND hWnd = FindWindow(NULL, "Calculator");

Syntax Reference : http://msdn.microsoft.com/en-us/library/aa929233.aspx

Example:

#include <windows.h>

int main()
{
HWND hWnd = FindWindow(NULL, "Calculator");
  if(hWnd == 0)
{
    MessageBox(0, "Error cannot find window.", "Error", MB_OK|MB_ICONERROR);
  }
else
{
                //Write your code to do somethings here
  }
  return 0;
}