not necessarily. since 2006 ive rebuilt my system totally except for the HDD.
ive had 4 different gpu's, added ram and recently changed my mobo and cpu and the only trouble i had was with the mobo. when vista booted up i got a message saying this is not a genuine vista product....yada yada. i simply reactivated windows using the free phone number and the automated system and all was good again.
dont get me wrong, if you change components then reinstall vista for whatever reason, your will have to reactivate again but there wont be a problem. but if your just replacing parts without reinstalling nothing will happen except with the mobo as i already said.
i think the one component you cannot get away with changing is your HDD, ive heard many not ever being able to use their oems again because of this change.
all this was with one single oem version of vista home premium 32bit.