Win32 program ensures that only one instance runs

Find the name of the current window class and the title to see if such a window handle exists. If such a window handle is found, the window handle is set to the front of the window.

HWnd = FindWindow (szWindowClass, szTitle );
If (hWnd)
{
// Set focus to foremost child window
// The "| 0x00000001" is used to bring any owned windows to the foreground and
// Activate them.
SetForegroundWindow (HWND) (ULONG) hWnd | 0x00000001 ));
Return 0;
}

The green section above: "| 0x00000001" is used to set the window with an existing handle to the top (at the top of the window Z axis) and activate the window.