Windows 8 is the operating system.  If you format the drive it, along with everything else will be gone.  I'm not up on 8 but if you upgraded to it I suppose there is a possibility that it is uninstallable and you can return to XP.  I've my doubts about that though.

I would save all my important files, uninstall my programs and some might require that you deactivate them.  I would suggest though that you use Belarc Advisor to note the Keys in use on each.

Then install XP and ALL the updates to get to SP3.