No need to reformat. You only need to reformat if you install a new motherboard chipset.
Not true, i think microsoft changed their mind or maybe from what i hear without confirmation you can active 3 time your oem vista. My board bios went out 1 month ago and i decided to change my board because i had many problem with it. So i changed my board but i was scared about reformat or not the os. Pop-up opened after 2 reboot to ask me to activate the os, i clicked Active and was succes. But i am scared again because i want to change my hard drive ( format is require ) i don't know if i can reactivate again or not. I think its when you change the CPU. Not sure