Only if you buy an OEM version of Vista do you need to do that, and even then only if you change motherboards. If you change motherboards, it means you've changed computers, and the OEM Vista license is meant to be only for 1 computer.
If you have the Retail (Full or Upgrade, Upgrade merely requiring an activated Windows XP install on your harddrive but allows a clean reformat install of Vista if you choose and costs less) then you have 5 computers, but only 1 computer at a time. That means it will activate on 5 different motherboards, assuming you stopped using the previous one. For people who upgrade their computers every year, retail is better than OEM, but in reality they will almost always just OK you if you have an OEM and call in to say your computer broke and you replaced the motherboard. If they don't let you, just call again, the next guy will let you.
XP had the same rules except the retail version had unlimited motherboards, as long as you only use 1 at a time. Vista caps it at 5 I think, but honestly who has gone through 5 motherboards for their home PC from the day they got XP until now? Maybe a handful at best.