I've said this before but it's worth repeating:

Many tears and hours of work can be saved when playing about with new operating systems or any other changes to an existing, working computer by buying a dual bay USB docking station and a spare HD as Clive has suggested. THE VERY FIRST THING YOU DO is use the docking station to make a clone of your currently working drive and put it somewhere safe. Now you can mess about to your heart's content, safe in the knowledge that it's all reversible.

I've used this process several times over the years and the cost is negligible in comparison to the time and anguish it saves.

How often have we all wanted there to be a nice big "UNDO LAST MISTAKE" button in our lives? For computers this is it.