Basically you just get your installation disc, do what I said earlier but when you get to choosing the partition (part of the drive) to install on, you would format the drive (clearing all data) and create a new, fresh partition with to install yourself a new OS on - make sure there is 2 partitions, a smaller 128mb or so one and a large one with the rest of the drive there.
This is what I've been trying to do but I can't get to the screen that let's you select these options. It keeps trying to send me to startup repair for some reason.