How to re-install on Formatted PC , still keep the same # of licences

I have subscription for 3 PCs,but only 2 are used. I will have to format one of those 2 PC, and install windows 7 again. In the past I would download an uninstall prog,and then install Norton again on the formatted PC, so I still use 2 licenses.  Now I can not see how do I do it and still keep only 2 licenses used ?

I have subscription for 3 PCs,but only 2 are used. I will have to format one of those 2 PC, and install windows 7 again. In the past I would download an uninstall prog,and then install Norton again on the formatted PC, so I still use 2 licenses.  Now I can not see how do I do it and still keep only 2 licenses used ?