3 PC licence position when upgrading OS

Apologies if this is a stupid newbie question, but couldn't find the answer in the help file.  I've bought the three-PC version of NIS2009, and installed it on (so far) two PCs.  I therefore have one licence left.  However, I'm going to be upgrading one of the PCs from Vista to Windows 7 when it comes out next month.  This will involve a reformat and a fresh OS install from scratch.  How will NIS licencing recognise that it's the same PC, and still show one licence left?

 

Hope you can set my mind at rest....!

 

Regards

John D