This is why I just bought two new Dell computers. It comes with all of the software installed and you just plug it into the wall and are ready to work. I've built these machines but I don't think it is even't cost effective. If your hard drive is four years old you are living on borrowed time anyway. Hard drives are dirt cheap. With rebates they can be bought for $0. I'd just replace it and would also consider just getting a complete unit down at Office Depot or someplace like that. Then you'll have everything you need. Take the other computer and toss it in the trash.
That would be a mistake.