[DllImport("user32.dll", SetLastError = true)]
 static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

 IntPtr hWnd = (IntPtr)FindWindow(windowName, null);

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool EnumChildWindows(IntPtr hwndParent, EnumWindowsProc lpEnumFunc, IntPtr lParam);

private List<IntPtr> GetChildWindows(IntPtr parent)
{
    List<IntPtr> result = new List<IntPtr>();
    GCHandle listHandle = GCHandle.Alloc(result);
    try
    {
         EnumWindowProc childProc = new EnumWindowProc(EnumWindow);
         EnumChildWindows(parent, childProc, GCHandle.ToIntPtr(listHandle));
    }
    finally
    {
         if (listHandle.IsAllocated)
               listHandle.Free();
    }
    return result;
}