If you like the tower and are satisfied with the way it looks and the space inside you can replace all hardware inside. You will need to know the form factor of the case and what form factor motherboard it can accomodate. You will then have a brand new computer.
From my experience OEM towers are basically bare bones crap. I would spend the money and just buy a new tower along with all the other hardware. As old as that machine is you will not want any of its hardware on your new build. That would bew like putting a lawn mower engine in a BMW.
You can totally rebuild by replacing everything in the tower. But you can pick out a nice tower that will add aesthetic value to your computer. I would start from scratch and build a new computer.
I have a 10 year old dell that hasn't been rebuilt in 8 years. It is a pile of crap and my father constantly says that all it needs is to be rebuilt. he thinks the computer is more like a car and that all parts are compatible to some extent. I need some good reasons to get a new computer. I can give you the exact specs if you need them. Any help at all would be greatly appreciated