Norton Anti Virus

I have a 3 licence version of Nortons Anti Virus with the tool installed on 3 PCs.   I need to reformat the HDD on one of these PCs and reinstall all of the software including Windows XP.  Can you tell me the process that I need to follow regarding the reinstallation of NAV as I am worried that I will get to the NAV install point and will be informed that the tool is already installed on 3 PCs.

Regards

Mike