I'll let someone else answer this as I am not an expert on Vista but with XP you could do a repair which allowed you to keep everything else as is.
However although I know people who have done this successfully with XP I personally never did it myself I always did a clean install because thats what I prefered to do.
Assumming that Vista is the same in this respect as XP then I see no reason that you cannot do this but if your copy of Vista is not a Full copy and is an OEM copy then it is my understanding that it may fail validation as its meant to die with the old computer and it may consider a new motherboard as a new PC. This was an issue that could happen with XP, not always but sometimes.
Not wanting to cloud the issue I am sure someone who is a Vista expert will confirm if there are any issues.