Microsoft cares most that you are using a valid copy of windows, and that it is being used on only one PC at a time.
They define a PC primarily by the motherboard and configuration. You should be able to reactivate on the same pc as many times as you wish. Remember, that when you load a clean copy of windows, you have a fully functioning system for 30 days before activation is required. That should be enough time to get everything right, including reloads if necessary. This process is not a problem if you have a legitimate copy and product key.