just as any software you have to buy licences for more then 1 PC if upgrading them all. I think Only Office you can put on multiple PC's office 2007Student edition I think has 3 allows. Office 2007 profressional is 1 (I know this I have tried it) you have to buy another key.
before I bought laptop I had built my old PC & had XP for it. then bought Vista 1 year later. Vista & XP are one PC only unless you buy another key.
Or even reinstalling or upgrading hardware if its over its 3x in XP you have to Call MS. to get a new key (Free) OR purchase a new one. Depends on what MS says.