Kind of a simple question without much information to go on, but I'll entertain it. There would be a couple indicators to let someone know when they would "need new parts". One, if the old parts fail to work and are either broken or burned out. Two, the old parts do not allow you to do the tasks you want it to do, ie: playing certain games or running certain programs. Other than that I think you need to be a little more specific with a problem you're having before anyone can really help you out.
BTW, Welcome to AT!