You have to partition the HD (hard drive) and don't do that until after you have saved to external media (external HD, zip or CD) important files and registry keys you want to keep.

To partition, if you don't know how to do it manually or on your own, I'd use an application, such as (but not necessarily) Partition Magic. (I don't want to give the appearance of endorsing it or any given product; I cited it as an example of an application that can provide you with the how-to info.)

P.S.: I forgot to ask do you have Linux ready to install? I have the Red Hat version, but haven't tried doing an install until I replace the drive on one of my 'puters. That way, as I see it, it's a clean slate and all the data I want retained would not risk being lost in the process. I know some have dual HDs - with a certain O/S or platform installed on one and, i.e., Linux on another. Because they were not tech. oriented and having dual drives created conflicts that had to be resolved, they ended up having to farm out their systems for the "fix."

I've not wanted to tinker with a system that works well. If it needs a revamp, I see it as: I'm out nothing even if I have to scratch everything a try again .