foreach (Process process in Process.GetProcessesByName("notepad"))
{
    if (process.Id != Process.GetCurrentProcess().Id) //Excludes the process the code is being run from
    {
        foreach (ProcessThread thread in process.Threads)
        {
            int HwND = process.MainWindowHandle.ToInt32();
            // use the handle however you need
        }
    }
}