Will I need to reinstall Vista if I uprade from my Pentium D processor to Quad core processor? Or can I just switch out the processors and keep running the same OS.
no, not really, depends, if somehow after the change you get error messages, then you have to repair. i think vista has advance enough to know when there's a processor change so you don't have to format or repair.
i did this once with xp and it worked fine , may not be the same with vista