The prefab are known as a "southern staircase" and are usually good only for carpet. I've cleaned a couple up and finished them and it still looks better than carpet, even if they are just some junk poplar. The problem comes in when you decide to put in oak treads/risers there is no stringers so once the old is removed there is nothing there. Kudos to Shawn, most people don't realize this.
The option in this case is a stair cap that goes over the existing tread and 1/8" oak plywood for the riser.
My suggestion would be remove carpet and staples and poly the treads and paint the riser either brown or black. Most people go with white riser but they get very beat up from the toe of your shoe.
If you decide on new oak treads/risers put 2 coats of poly on them before you even get a saw out. After that, cut and install then lightly sand w/220 clean and coat one last time.
