system beeps and slow down

My pc beeps and slows down at least 15 minutes after turn it on. Sometimes it will not beep for a couple of days then it start beeping again. Even if I press the reset button or restart the computer from windows it will not stop beeping. What I do now is to turn it off for about five minutes and turn it back on then it will not beep for sometime and all of a sudden it start beeping slow down again. Please help.