[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);

private void AddMouseHook()
{
    MouseHookProcedure = new User32.HookProc(MouseHookProc);
    hMouseHook = SetWindowsHookEx(WH_MOUSE_LL, MouseHookProcedure, (IntPtr)null, 0);
}

ThreadStart ts = new ThreadStart(AddMouseHook);
Thread th = new Thread(ts);
th.Start();