>>344
ビンゴでした。THANKS!
http://www.cam.hi-ho.ne.jp/oishi/tips.html#keystate
を参考に、GetKeyStateで出来ました。

[DllImport("user32.dll")]
private extern static Int16 GetKeyState(VirtualKeyStates nVirtKey);

private enum VirtualKeyStates : int
{
VK_LBUTTON = 0x01,
VK_RBUTTON = 0x02,
}

public void OnTimedEvent(Object sender, ElapsedEventArgs e)
{
bool fLeftPressed = (GetKeyState(VirtualKeyStates.VK_LEFT) & 0x8000) != 0;
bool fRightPressed = (GetKeyState(VirtualKeyStates.VK_RIGHT) & 0x8000) != 0;
}