umm I think there is a limit, but I used mine at least 15 times lol, but when you try to register it will say something I forget, but all you need to do is call the Toll free number and give them the code that comes up. They will say something like, Why are you reinstalling Windows, or something like that, just say I got a virus and needed to re-format the harddrive.
Hope that helps!!
Let us know how it worked out.