Penumbra is not really a graphic intensive game. Yes it is worth trying an alternative to your mouse/keyboard. At the moment what we are trying to do is eliminate possible causes no matter how odd they might seem.''DirectInput' I believe is the cause of your gaming problems. So what you have attached to your PC will have some bearing. Silly question but you did install all your motherboard drivers when you installed XP?
By the way if you like Penumbra, then have a look at this.http://www.amnesiagame.com/#main