One question: How full is your hard drive? When mechanical drives get very full (over ~80% of capacity) you can sometimes see them start to struggle with longer seek times, load times, etc.
Given the age of the drive and the problems you're experiencing, though, it's entirely likely that you should look at replacing it. Since you're on W8, I doubt it's Windows rot since you must've done a fresh install fairly recently. All of the advice in this thread so far is good.