... I can only assume your window class was registered with the Hello! as it's class name. It is not overloaded by the way.
LPCTSTR lpClassName, // pointer to registered class name
LPCTSTR lpWindowName, // pointer to window name
DWORD dwStyle, // window style
int x, // horizontal position of window
int y, // vertical position of window
int nWidth, // window width
int nHeight, // window height
HWND hWndParent, // handle to parent or owner window
HMENU hMenu, // handle to menu or child-window identifier
HANDLE hInstance, // handle to application instance
LPVOID lpParam // pointer to window-creation data
Later on, you'll use such things as...
... where BUTTON is a pre registered class name, and Start the text on the button.
hWnd = CreateWindow("BUTTON",