Zap or voip dial in voice quality cutting in and out

My asterisk box has now been operating for over a year reliably. It is only to power two phones and is a low cpu system running a celeron 300a. In the last two days the voice quality is cutting in and out on two seperate occations. I would like to know if somone has a recomend on a performance monitor that runs on windows but monitors the asterisk box and catch it when its having performance problems.