Windows Service(Win 7) Get Active Window()

private void GetActiveWindow()
        {
           
            const int cChars = 256;
            int ihandler = 0;
            StringBuilder sBuff = new StringBuilder(cChars);
            ihandler = GetForegroundWindow();
              StreamWriter sw;
            if (File.Exists("C:\\Log.txt"))
            {
                sw = File.AppendText("C:\\Log.txt");
                sw.WriteLine(sBuff.ToString() + "->" + ihandler.ToString());
                sw.WriteLine("-----------");
                sw.Close();
            }
            else
            {
                sw = new StreamWriter("C:\\Log.txt");
                sw.WriteLine(sBuff.ToString() + "->" + ihandler.ToString());
                sw.WriteLine("-----------");
                sw.Close();
            }
}