It really depends on how many times you have activated it. Its no big deal if it doesn't go through. I've only had to call the number a few times with XP and you'll enter a big code. 80% go through the other 20% you'll spend activating with a live person. They just ask you why you formatted just say you got a virus and need to reformat.
DO NOT TELL THEM YOU GOT ANY NEW HARDWARE! A simple answer like I got a virus or I was getting errors and thought a reformat would cure it.